Two double-blind, crossover trials comparing the antiemetic effectiveness of nabilone, a new synthetic cannabinoid, with that of prochlorperazine were conducted in patients with severe nausea and vomiting associated with anticancer chemotherapy. Of 113 patients evaluated, 90 (80 per cent) responded to nabilone therapy, whereas only 36 (32 per cent) responded to prochlorperazine (P less than 0.001). Complete relief of symptoms was infrequent, occurring only in nine patients (8 per cent) given nabilone. When both drugs were compared, both nausea (P less than 0.01) and vomiting episodes (P less than 0.001) were significantly lower in patients given nabilone. Moreover, patients clearly favored nabilone for continued use (P less than 0.001). Predominant side effects noted by patients were similar for both agents and included somnolence, dry mouth and dizziness but were about twice as frequent and more often severe in patients receiving nabilone. In addition, four patients (3 per cent) taking nabilone had side effects (hallucinations in three, hypotension in one) that required medical attention. Euphoria associated with nabilone was infrequent (16 per cent) and mild.