A prospective randomized double-blind trial comparing the butyrophenone analogue domperidone (D) and the synthetic cannabinoid nabilone (N) in the treatment of cytotoxic-induced emesis was conducted in 38 patients receiving highly emetogenic chemotherapy regimens (70% containing cisplatin). Patients received 20 mg D or 1 mg N the night before chemotherapy and 8-hourly on each chemotherapy day for two consecutive cycles of treatment. Three of 19 patients randomized to N completed only one cycle because of disease progression or subjectively adverse effects. Four of 19 patients completed only one cycle of D because of lack of efficacy or chemotherapy toxicity. In all, 32 cycles of N and 33 cycles of D were evaluable for efficacy. The mean number of vomiting episodes in cycle 1 was 4.76 for N and 12.95 for D (P less than 0.02). The corresponding values for cycle 2 were 4.27 and 7.69 (P greater than 0.10), and for cycles 1 and 2 combined, 4.53 for N and 10.81 for D (P less than 0.01). Nausea and food intake scores did not differ significantly, although there was a trend towards less nausea and an increased food intake with N. Subjectively adverse effects were more frequent with N and included drowsiness, dizziness, dry mouth, and postural hypotension. N is superior to D for the control of cytotoxic-induced emesis.