The antiemetic activity and side-effects of delta-9-tetrahydrocannabinol (THC) were evaluated in 116 patients (median age 61 years) receiving combined 5-fluorouracil and semustine (methyl CCNU) therapy for gastrointestinal carcinoma. In a double-blind study, patients were randomized to receive THC, 15 mg orally three times a day, prochlorperazine, 10 mg orally three times a day, or placebo. The THC had superior antiemetic activity in comparison to placebo, but it showed no advantage over prochlorperazine. Central nervous system side-effects, however, were significantly more frequent and more severe with THC. With the dosage and schedule we used, and in our patient population of largely elderly adults, THC therapy resulted in an overall more unpleasant treatment experience than that noted with prochlorperazine or placebo. Although THC may have a role in preventing nausea and vomiting associated with cancer chemotherapy, this role must be more clearly defined before THC can be recommended for general use.