The antiemetic effect of oral nabilone, a synthetic cannabinoid, given at a dose of 2 mg every 12 hours was compared to oral slow-release capsules of prochlorperazine given at a dose of 10 mg every 12 hours by a double-blind crossover method in 37 patients receiving cancer chemotherapy. Patients received one of the following as the primary emetic stimulus: high-dose cis-dichlorodiammineplatinum(II) (DDP), low-dose DDP, mechlorethamine, streptozotocin, actinomycin D, or DTIC. Although results varied according to strength of emetic stimulus received, both nabilone and prochlorperazine appeared to produce antiemetic effects. Eighteen of the 37 patients achieved a complete or partial elimination of symptoms: seven with nabilone alone, three with prochlorperazine alone, and eight with each drug. Nabilone appeared to be the more effective antiemetic for patients who received chemotherapy agents other than high dose DDP; it was equivalent to prochlorperazine for those who did receive high-dose DDP. Side effects from prochlorperazine were limited to mild drowsiness occurring among 35% of the patients. The side effects from nabilone were drowsiness and dizziness which occurred frequently and were dose-limiting in 25% of patients.