Fifty-five patients harboring a variety of neoplasms and previously found to have severe nausea or emesis from antitumor drugs were given antiemetic prophylaxis in a double-blind, randomized, crossover fashion. Tetrahydrocannabinol (THC), prochlorperazine, and placebo were compared. Nausea was absent in 40 of 55 patients receiving THC, eight of 55 patients receiving prochlorperazine, and five of 55 in the placebo group. The antiemetic effect of THC appeared to be more efficacious for cyclophosphamide, fluorouracil, and doxorubicin hydrochloride, and less so for mechlorethamine hydrochloride and the nitrosureas. Tetrahydrocannabinol appears to offer significant control of nausea in most patients and exceeding by far that provided by prochlorperazine.