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. delta 9-Tetrahydrocannabinol (THC), prochlorperazine, and placebo were compared. Nausea was absent in 40 of 55 patients receiving THC, in 8 of 55 patients receiving prochlorperazine, and in 5 of 55 in the placebo group. THC appeared to be more efficacious in controlling the emesis associated with cyclophosphamide, 5-fluorouracil, and doxorubicin and less so for nitrogen mustard and the nitrosourea. THC appears to offer significant control of nausea in most patients and exceeds by far that provided by prochlorperazine (P less than 0.005).