An anti-emetic drug, nabilone, a synthetic cannabinoid, has been compared with prochlorperazine in 24 lung cancer patients receiving cancer chemotherapy. Each of the drugs studied was given orally every 12 hours, starting the night before chemotherapy, during one of two consecutive identical chemotherapy cycles in accordance with a double-blind cross-over random order assignment. Single doses were 2 mg of nabilone, or 15 mg of prochlorperazine. The chemotherapeutic regimens given included the following drugs in various combinations: cis-platinum, vincristine, cyclophosphamide, adriamycin, vindesine, and etoposide (VP16). Nabilone was significantly superior to prochlorperazine in the reduction of vomiting episodes. Side effects, mainly vertigo, were evident in nearly half of the patients after nabilone, and three patients were withdrawn from the study due to decreased coordination and hallucinations after nabilone. Side effects from prochlorperazine were limited to mild drowsiness in one patient. Two-thirds of the patients preferred nabilone to prochlorperazine. We conclude that nabilone is a moderately effective anti-emetic drug, but that the unpredictability of its side effects call for careful patient information, especially with elderly outpatients. We recommend that at least after the first dose of nabilone, the patient should be kept under close observation during 4 hours.