Delta-9-tetrahydrocannabinol (THC) is an effective antiemetic as compared with placebos in patients receiving chemotherapy for cancer. In this study we compared THC with prochlorperazine (compazine) in a randomized, double-blind, crossover trial with patients who had failed to benefit from standard antiemetic therapy. Regardless of the emetic activity of the chemotherapeutic agents, there were more complete responses to THC courses (in 36 of 79 courses) than to prochlorperazine (in 16 of 78 courses). Of 25 patients who were treated with both drugs and who expressed a preference, 20 preferred THC (P = 0.005). Among patients under 20 years of age there was a higher proportion of complete responses to THC courses (15 of 20) than among older patients (21 of 59 courses; P = 0.004). Increased food intake occurred more frequently with THC (P = 0.008) and was associated with the presence of a "high." Of 36 THC courses resulting in complete antiemetic responses, 32 were associated with a high. We conclude that THC is an effective antiemetic in many patients who receive chemotherapy for cancer and for whom other antiemetics are ineffective. (N Engl J Med 302:135--138, 1980).