Metoclopramide has been used for almost 40 yr to prevent postoperative nausea and vomiting (PONV). We have reviewed the efficacy and safety of metoclopramide for the prevention of PONV. A systematic search (MEDLINE, EMBASE, manufacturers' databases, hand searching, bibliographies, all languages, up to June 1998) was performed for full reports of randomized comparisons of metoclopramide with placebo in surgical patients. Relevant end-points were prevention of early PONV (within 6 h after operation), late PONV (48 h) and adverse effects. Combined data were analysed using relative benefit/risk and number-needed-to-treat/harm. In 66 studies, 3260 patients received 18 different regimens of metoclopramide, and 3006 controls received placebo or no treatment. There was no evidence of dose-responsiveness with oral, i.m., intranasal or i.v. metoclopramide in children and adults. In adults, the best documented regimen was 10 mg i.v. There was no significant anti-nausea effect. The numbers-needed-to-treat to prevent early and late vomiting were 9.1 (95% confidence intervals 5.5-27) and 10 (6-41), respectively. In children, the best documented regimen was 0.25 mg kg-1 i.v. The number-needed-to-treat to prevent early vomiting was 5.8 (3.9-11). There was no significant late anti-vomiting effect. Minor drug-related adverse effects (sedation, dizziness, drowsiness) were not significantly associated with metoclopramide. There was one adult who experienced extrapyramidal symptoms with metoclopramide.