Oral delta-9-tetrahydrocannabinol (THC), 15 mg/m2, was compared to prochlorperazine (PCZ), 10 mg. for the control of cancer chemotherapy-related emesis. Thirty-six patients whose vomiting was refractory to standard antiemetic therapy were entered in this randomized comparative cross-over study. THC decreased nausea and vomiting in 23 of 36 (64%) patients compared to 1 of 36 receiving PCZ. THC efficacy was not dependent on the class of antineoplastic-agent inducing the emetic symptoms, age of patients or type of sensorial change experienced. Using the 15 mg/m2 dose, all patients experienced transient sensorial changes, characterized as a pleasant "high" in 19 or a variable state of dysphoria in 17 cases. This study confirms the usefulness of THC in patients whose chemotherapy-induced nausea and vomiting is refractory to other standard antiemetics. While excellent antiemetic control was achieved at the dosage 15 mg/m2, dysphoria was encountered at this dose level and we recommend that an initial dose of 5 mg/m2 which, if necessary, can be carefully increased to achieve maximum antiemetic benefit.