The aim of our study was to find a therapeutic range for the serum concentration of zuclopenthixol (S-Zu) in chronic schizophrenic patients. S-Zu was measured in 17 patients and dosage reduction was suggested by the laboratory if S-Zu exceeded 15 nmol/L. The clinical symptoms and side effects were evaluated blindly using the Brief Psychiatric Rating Scale (BPRS) and the UKU rating scale, respectively. S-Zu and ratings were repeated 6, 12, and 24 weeks after the final dosage adjustments. In 7 of 10 patients with S-Zu > 15 nmol/L the dosage was reduced by 20-67%. After dosage reduction the S-Zu was below 15 nmol/L in 10 of the 17 patients. The mean BPRS score and the side effects, evaluated by the UKU scale, were reduced in patients in whom the dosage was reduced. It is suggested that S-Zu in the range 5-15 nmol/L may serve as a preliminary therapeutic range for S-Zu.