Our objective was to compare the physical health status of adult prisoners with and without a mental illness. Mental illness was diagnosed in a sample of 557 Australian prisoners using the Composite International Diagnostic Interview (CIDI). Physical health measures included self‐reported chronic health conditions, recent health complaints and symptoms, self‐assessed health using the Short‐Form 36 Health Survey (SF‐36), and markers of infectious diseases known to be highly prevalent among prisoner populations (hepatitis A, hepatitis B, and hepatitis C). Men and women with a mental illness had lower scores on the SF‐36 compared with those without a mental illness indicating poor overall health. Adjusting for age and sex, a diagnosis of any mental illness (symptoms of psychosis, anxiety or affective disorder) was positively associated with a history of head injury, back problems, asthma, peptic ulcers, cancer, and epilepsy/seizures. There was a significant association between post traumatic stress disorder and asthma, a history of head injury, peptic ulcers, and cancer. There was no significant difference in the proportion of current tobacco smokers in the mentally ill and nonmentally ill groups (81% vs. 77%, p = 0.33). However, those with a mental illness were less likely than those with no diagnosis to exercise in the past 4 weeks (79% vs. 89%, p = 0.002). Mentally ill prisoners also have significant physical co‐morbidity compared with the non‐mentally ill. These findings suggest that those providing mental health services to prisoners should adopt a holistic approach embracing both mental and physical health.