A study was carried out in 68 chronic schizophrenic out-patients to assess the therapeutic effectiveness and tolerance of long-term maintenance treatment with a depot preparation of clopenthixol decanoate. Patients received 200 mg to 600 mg doses by intramuscular injection every 2 to 3 weeks for a mean treatment period of 20.3 months. Clinical symptomatology was assessed at regular intervals using the Brief Psychiatric Rating Scale (BPRS) and any adverse effects were recorded against a standard check-list. The results showed that there was a significant reduction in total and factor composition BPRS scores after treatment in all patients, the improvement becoming generally evident by the sixth month of treatment. Few autonomic and neurological side-effects were reported.