SUMMARYA four-week double-blind study of zuclopenthixol and thioridazine in 64 elderly patients presenting with hostility or restlessness has been carried out in five Finnish mental hospitals. The ages of the patients in the two groups ranged from 64 to 97 years. The initial daily dose was 2-6 mg zuclopenthixol or 20-60 mg thioridazine. In week 4 the mean daily dose was 6.8 mg zuclopenthixol and 62 mg thioridazine. After one week of treatment the severity of illness was reduced significantly in the zuclopenthixol group and in both groups after two and four weeks. The key symptom hostility showed significant improvement in both treatment groups after only one week of treatment. The other key symptom restlessness was also significantly improved in both groups. The only significant difference between the groups was a better effect on sleep disturbance with zuclopenthixol. Few side-effects were recorded in either group, and no serious effects on cardiovascular function were found. The study indicates that both zuclopenthixol and thioridazine are effective drugs in the treatment of elderly patients with symptoms such as hostility, aggressiveness and restlessness.