BACKGROUND: To investigate the dietary adequacy of prisoners of Beon Prison, Madang, Papua New Guinea in response to a report of possible nutritional deficiency. METHODS: We undertook an observational, cross-sectional study. All 254 male inmates (May 2010) were eligible to answer a validated interview-based questionnaire; to have a comprehensive dietary assessment; and to provide blood for biochemical analysis (alpha-tocopherol, beta-carotene, lutein, thiamin, riboflavin, niacin, folate, homocysteine, zinc, ferritin, and vitamins A, B12 and C). Prison guards were invited to participate as a comparison group. RESULTS: 148 male prisoners (58.3%) and 13 male prison guards participated. Prison rations consisted of white rice fortified with thiamin, niacin, and iron, tinned tuna, tinned corned beef, water crackers, and black tea, with occasional intakes of fruit and vegetables. Some prisoners received supplementary food from weekend visitors. From assessment of the prisoners dietary data, median intakes of calcium (137 mg), potassium (677 mg), magnesium 182 mg), riboflavin (0.308 mg), vitamin A (54.1 mug), vitamin E (1.68 mg), vitamin C (5.7 mg) and folate (76.4 mug) were found to be below estimated average requirements (EAR).Following are the prisoners median (P25, P75) concentration of circulating nutrients and the percentage of prisoners with levels below normal reference ranges or recognized cut-off values: serum retinol 0.73 (0.40, 1.21) mumol/L, 46% below 0.7 mumol/L; plasma folate 2.0 (1.4, 2.6) nmol/L, 98% below 6.8 nmol/L; plasma vitamin C 6.3 (1.0, 19.3) mumol/L, 64% below 11.4 mumol/L; serum zinc 9.9 (8.8, 11.1) mumol/L, 66% below 10.7 mumol/L. Guards had diets with a higher dietary diversity that were associated with greater intakes of nutrients and biomarker concentrations. CONCLUSIONS: The prisoners diets are likely lacking in several micronutrients and recommendations for dietary change have been made to the prison authorities. Ongoing vigilance is required in prisons to ensure the basic human right of access to a nutritionally adequate diet is being observed.