Nabilone, a synthetic cannabinoid, and Prochlorperazine were compared in a double-blind crossover study of 34 patients with lung cancer undergoing a 3-day schedule of chemotherapy with Cyclophosphamide, Adriamycin and Etoposide. Symptom scores were significantly better for patients on nabilone for nausea, retching and vomiting (P less than 0.05). Fewer subjects vomited with nabilone (P = 0.05) and the number of vomiting episodes was lower (P less than 0.05); no patients on nabilone required additional parenteral anti-emetic. More patients preferred nabilone for anti-emetic control (P less than 0.005). Adverse effects common with nabilone were drowsiness (57%), postural dizziness (35%) and lightheadedness (18%). Euphoria was seen in 14% and a "high" in 7%. Erect systolic blood pressure was lower in nabilone patients on Day 1 (P = 0.05) but postural hypotension was a major problem in only 7%. Nabilone is an effective oral anti-emetic drug for moderately toxic chemotherapy, but the range and unpredictability of its side-effects warrant caution in its use.