Two companion post-marketing studies have evaluated the frequency of adverse events amongst patients receiving atracurium. In this first report, we describe the study methods used in both centres and the findings of the study conducted at the Group Health Co-operative of Puget Sound, U.S.A. In this study, the frequency of adverse events in 1013 patients receiving atracurium was substantially the same as that in 851 patients receiving other neuromuscular blocking agents. In both groups there were no instances of cardiac arrest, anaphylaxis or death, no adverse effects associated with failure to antagonize neuromuscular block, and no instances of previously unreported types of adverse effects. We conclude that the safety profile of atracurium is similar to that of other neuromuscular blocking agents and that this type of observational study of anaesthetic agents may prove a useful tool for surveillance of new drugs used primarily in hospitals.