During clinical experience with the "atypical" neuroleptic drugs clozapine, risperidone, and zotepine, some patients have shown a marked weight gain. To prove whether weight gain is a relevant side effect of atypical neuroleptics, the charts of all patients admitted with DSM-III-R diagnoses of schizophrenia, schizoaffective disorder, or delusional disorder in the years 1991 to 1995 were evaluated. A retrospective chart review was performed, which included all patients who were treated longer than 2 weeks with a single neuroleptic. The data analysis showed that weight gain must be considered as a common side effect of atypical neuroleptics (clozapine, risperidone, sulpiride, or zotepine). The mean weight gain (3.1, 1.5, 1.9, or 4.3 kg, respectively) was significantly higher than that of patients treated with "classic" neuroleptics (mean, 0.0-0.5 kg) (Kruskal-Wallis, p = 0.01). Young and not obese patients show the highest weight increase. Because weight gain occurs in the first weeks of treatment, particularly in previously untreated subjects, this side effect has to be considered in view of compliance with long-term neuroleptic medication.