Objective To collect information on health determinants directly from prisoners, complementary to a needs assessment. Design Self-completion multiple-choice questionnaire to a sample based on alternate cells. " Setting Her Majesty's Prison (HMP) Cardiff. Method Three hundred men received questionnaires with an offer of confidential help, which was accepted by two. Questions included qualifications, previous occupation, drug, alcohol and medical history, smoking, perceived threats, worries, diet, exercise, drugs in prison, access to services, and the Hospital Anxiety and Depression (HAD) scale. Results Of 133 (44 per cent) who responded, more than half had left school prematurely. Thirty-three (25 per cent) drank 90 alcohol units or more weekly before prison and 91 (68 per cent) used illegal drugs, with 44 (33 per cent) using in prison. Of the 112 (84 per cent) who smoked, 89 (79 per cent) wished to quit. Ninety-one (68 per cent) never took vigorous exercise, and 83 (62 per cent) ate less than three portions of fruit and/or vegetables daily. Only 41 (35 per cent) were within the normal HAD range for both anxiety and depression. Conclusion This study highlights concurrent high levels of adverse health determinants in prisoners. Targeting these determinants should improve health and decrease the chance of returning to criminality on release.