The present prospective study describes the demographic, medical and psychosocial characteristics of 40 people with intellectual disability who were referred for psychiatric inpatient treatment in the special psychiatric unit of the Special Welfare District of Southwest Finland. Three different control groups were used to study: (1) demographic variables (n = 122); (2) medical history (n = 39); and (3) psychosocial factors (n = 20). The symptoms leading to an admission to inpatient care and the connections of these clinical signs with the discharge diagnosis were evaluated. The typical inpatients were young males with mild intellectual disability, psychosis and a previous psychiatric diagnosis. They had lived in several places during their lives and their economic situation was poor. Affective and/or disruptive symptoms were the most common causes of an admission to inpatient care. The largest diagnostic group at discharge consisted of patients with psychotic disorders. The people with intellectual disability who were admitted for inpatient care formed a subgroup with certain psychiatric symptoms and social problems. Specialist psychiatric expertise is absolutely necessary for the treatment of this subgroup.