Abstract Purpose To determine the incidence and characteristics of mental disorders (MD) in the Intensive Care Unit (ICU), and to define a classification system adapted to the ICU environment. Design A retrospective, descriptive analysis. Setting Intensive Care Unit, Arnau de Vilanova Hospital in Lérida (Spain). Patients All patients with MD admitted during 5-year period (January, 1 2004 to December 31, 2008). Main variables General variables included clinical-demographic data, diagnostic variables, procedures, severity score, length of stay and mortality. Specific variables included psychiatric history, screening for substance abuse, psychiatric assessment, monitoring and transfer to a psychiatric center. Classification of the MD was as follows: (1) acute substance intoxication (SI); (2) suicide attempts (SA); and (3) MD associated with the main diagnosis (AMD). Results A total of 146 patients had MD (7.8%); they were predominantly male (74%) and were younger than the general ICU population (43.9 years vs 55.3 years, p<0.001). The ICU stays of the patients with MDs were shorter (4 days vs 7 days, p<0.001), and there was less hospital mortality (17.1% vs 25%, p<0.05). They also showed a higher incidence of pneumonia (19.9% vs 13.8%, p<0.05), but no differences in the level of severity were observed. The SI group (24.7%) contained the highest number of young people; the SA group (36.3%) showed a predominance of women; and the AMD (39%) group had the longest stays and the highest mortality. Psychiatric consultation was carried out mainly in the SA group (62.3%). Conclusions MD is a relatively common problem in the ICU. Collaboration with the Psychiatry Department seldom occurs, but must be encouraged to develop fully integrated management of critical patients with MD.