Nineteen chronic schizophrenic in-patients were entered into a 12-week open study of intramuscular clopenthixol decanoate. Fourteen patients completed the full 12-week treatment period; 3 were withdrawn due to inadequate clinical response or untoward adverse effects, and 2 were discharged and lost to follow-up. Doses administered were in the range 200 to 400 mg, weekly or fortnightly. Patients were assessed weekly using the Brief Psychiatric Rating Scale (BPRS). Nurses' Observation Scale for In-patient Evaluation (NOSIE-30) and the Clinical Global Impression (CGI). Unwanted effects were recorded on a checklist. Statistically significant improvements in the overall mean scores were noted on the BPRS at every assessment and on the NOSIE-30 at every assessment except in Week 4. Unwanted effects were generally mild and they decreased during the study. The results suggest that clopenthixol decanoate is an effective, well-tolerated antipsychotic agent, suitable for the treatment of chronic, long-stay schizophrenic in-patients.