A prospective, randomized and double-blinded trial of the comparative effects of delta-9-tetrahydrocannabinol (THC) and haloperidol (H) was begun in February 1980. Patients were randomized to initially receive either THC or haloperidol with cross-over to the other agent after two courses. All patients evaluated efficacy and toxicity of each agent and those patients completing the study expressed a preference for either THC or haloperidol. All patients are receiving chemotherapeutic agents known to induce severe vomiting (cis-platinum, nitrogen mustard, or doxorubicin) or have a history or retching with chemotherapy. Fifty-two patients are evaluable as of October, 1980. THC and haloperidol were equally effective in controlling nausea and vomiting as judged by number of vomiting episodes, patient evaluation of efficacy, and patient preference. About 10% of patients had complete control of vomiting and a third had less than five episodes. Patients failing one of the antiemetics had good control with the other about half the time. Toxicities from THC were less well tolerated than those from haloperidol, but most patients had no serious side effects. Nonoverlapping toxicities and efficacy raise the possibility that a combination of the agents might be worthwhile.