Since 2000, ophthalmologists providing clinical services in Madang, Papua New Guinea, have become aware of local adult prisoners presenting with gradual vision loss. An informal assessment conducted in Beon Jail in 2007 indicated that at least 40 local inmates had a significant level of visual impairment. This was most commonly associated with atrophy of the optic nerves, a result of optic neuropathy (ON) and suspected to be caused by nutritional deficiency. A more detailed assessment of the prison population including clinical characterization of the ON, comprehensive dietary analysis and assessment of toxic risk factors was necessary to help determine disease aetiology and appropriate treatment for prisoners. In May 2010, a collaborative project between the Fred Hollows Foundation and the University of Otago was made to further investigate the occurrence of ON previously documented in Beon Prison. All adult prisoners detained in Beon Prison (264 persons) were invited to participate in the study. Beon prison guards were also invited to participate on the basis of some shared environmental exposures with prisoners. Consenting participants were individually interviewed regarding demography, general and ocular health, diet and lifestyle. Participants underwent a vision and ocular examination as well as a physical examination performed by a team of trained health workers. A 24-hour dietary recall interview was performed on each participant and a venous blood sample was collected by venipuncture for the analysis of nutrients and other biomarkers. As an additional measure of dietary intake, samples (n=30) of prison rations were weighed and analysed for nutrient content. Finally, prison food samples were analysed for lead and cadmium content.Consent was obtained for 158 prisoners and 17 prison guards. Due to a low female contingent in the prison, only data collected from males was analysed. ON was found to be present in the prisoner population. Of the prisoners examined (n=135), 14 had 'definite' or 'likely' ON and 30 had 'possible' ON. No guards had 'definite' or 'likely' ON. No cases of peripheral neuropathy were found in participants. The prisoner diet predominantly consisted of rice, canned corned beef, canned tuna, water-crackers, tea and water. The fruit and vegetable consumption of the prisoners was low - 66% reported to never or rarely consume fruit and vegetables whilst in prison. Nutrient intake data generated from the 24 –hour recall suggested that less than 25% of prisoners met the estimated average requirement (EAR) for vitamin A, folate, vitamin C, vitamin E, potassium and calcium. Biochemical assessment indicated that over half of the prisoners fell below the cut-offs values of deficiency for biochemical indicators of vitamin A, folate and vitamin C. An elevated plasma homocysteine concentration was also found in 79% of prisoners. Blood concentrations of α-tocopherol, thiamin, B12 and selenium predominantly fell within normal ranges (<5% below recommended cut-offs).Both age and time of incarceration in Beon Prison were significantly correlated with ON severity. A significant inverse trend between ON severity was found with both whole blood folate and red blood cell folate concentrations using linear regression when adjusted for age and time of incarceration. Investigation into possible toxic causes of the disease showed a low exposure of prisoners to common risk factors of ON including alcohol, cassava, and lead. Other risk factors including tobacco and the use of certain pharmaceutical drugs were not correlated to disease risk.Our findings suggest that folate deficiency has a likely aetiological role in the ON found in this prisoner population. The involvement of other B-vitamins, nutrient deficiencies or toxic agents may however be causal or contributing to the onset and development of ON. This investigation highlighted nutritional ON in prisoners of Beon jail, Madang. As a response to these findings recommendations for multivitamin supplementation and dietary changes including the addition of fruit, leafy green vegetables and milk powder to the prison diet have been made to the prison authorities in order to correct nutritional deficiencies and to prevent further cases of ON.