CSF 5-HT and 5-HIAA were measured in endogenously depressed patients (ICD-9) (n = 23) and controls (n = 11). Distribution of sex, age and body height was similar in the two groups. Non-parametric statistics were used. In depressed patients CSF 5-HT concentrations were found to be higher (P less than or equal to 0.01) than in controls. A further classification of the depressed patients by the Newcastle Scale showed that the highest values were found in the endogenous group compared to the non-endogenous group (P less than or equal to 0.02). CSF 5-HIAA was found to be equal in the two groups, even when pairs matched for height were compared. No relation between clinical recovery due to drug treatment and changes in CSF 5-HT was seen. Our data support a possible involvement of 5-HT in the biology of depression, but the anatomical and functional levels of a serotonin derangement are still unknown.