The post-operative anti-emetic efficacy of 4 and 8 mg of ondansetron in adult patients with and without a previous history of motion sickness (PHMS) was assessed by meta-analysis. MEDLINE and EMBASE databases were searched for randomized placebo-controlled trials evaluating the anti-emetic effectiveness of ondansetron in a 24-h period. In the 49 studies found, a further selection was with respect to those studies that noted the patient's previous history of motion sickness. Twelve trials involving 2122 patients; 446 previous history of motion sickness(+)patients and 1676 previous history of motion sickness(−) patients met the selection criteria. The dose of 4 mg ondansetron was 71.5% more effective in previous history of motion sickness(+) than in previous history of motion sickness(−) patients. For the 8 mg dose, the odds ratios (95% CI) were: previous history of motion sickness(+)=3.11 (1.40–6.93) and previous history of motion sickness(−)=2.08 (1.35–3.21). The calculated number needed to treat was also more favourable in previous history of motion sickness(+) patients for both doses of ondansetron,demonstrating a higher effectiveness in this subgroup of patients.