Objective: This study examined demographic and clinical characteristics of frequent users of a psychiatric inpatient unit in Portugal. Methods: Data (2004-2008) for 1,348 consecutive psychiatric inpatients were reviewed. Frequent users (N=137), who had at least three admissions in the study period, were compared with non-frequent users (N=1,211) on age, gender, race-ethnicity, diagnosis, and compulsory admissions. Data were analyzed with chi square and Student's t tests. Results: Frequent users accounted for 29\% of admissions. They were significantly younger than non-frequent users (39 +/- 14 versus 44 +/- 17, p<.001), and a larger proportion had compulsory admissions (28\% versus 14\%, p<.001). The frequent user group also had significantly higher rates of bipolar disorder (61\% versus 46\%, p<.001) and schizophrenia (29\% versus 18\%, p<.003). Conclusions: Understanding characteristics of frequent users can inform development of appropriate services. Research should address other variables related to frequent admissions, including socioeconomic factors, general medical and psychiatric comorbidities, and treatment compliance.