The objective was to compare the incidence of adverse reactions reported with three nonsteroidal anti-inflammatory drugs with different cyclo-oxygenase (COX)-2 selectivity. All spontaneous adverse reaction notifications in the pharmacovigilance database of the World Health Organisation Collaborating Centre for International Drug Monitoring with aceclofenac, meloxicam, and rofecoxib that were recorded during the first year of marketing were included. The incidence rate (adverse reactions/10(6) defined daily dose) and 95% confidence interval for total adverse reactions was 8.7 (6.1-12.0) for aceclofenac, 24.8 (23.1-26.6) for meloxicam, and 52.6 (49.9-55.4) for rofecoxib. Aceclofenac had a lower incidence of gastrointestinal bleeding, abdominal pain, and arterial hypertension than meloxicam and a lower incidence of gastrointestinal bleeding, abdominal pain, liver toxicity, thromboembolic cardiovascular events, arterial hypertension, and edema than rofecoxib. The incidence of total and gastrointestinal adverse reactions was significantly lower with aceclofenac than with meloxicam or rofecoxib, thus raising doubts about the hypothetical advantage of COX-2 selective inhibitors.