The characteristics of mental disorders, as well as deficiencies in their treatment, must be properly defined. This was a prospective, longitudinal, observational study, in which all men referred to a penitentiary psychiatric consultation of three penitentiary centers in Spain were invited to participate. Those who consented to participation (1328) were interviewed at the baseline timepoint and at intervals for up to 3 years. The presence of mental disorders was high: 68.2% had a cluster B personality disorder, 14% had an affective and/or anxiety disorders, 13% had schizophrenia, and over 80% had a dual disorder. Polypharmacy was the norm. Moreover, the health care received in prison did not match that provided in the community in terms of quantity and quality. These results should help to facilitate the design of mental healthcare provision for prisoners, focusing on both the most frequent patient profiles and equality of care.