The French network of Regional Pharmacovigilance Centres evaluated in 1997 the prevalence of adverse effects of drugs (AED). In 1998, and again with the support of the French Drug Agency, in collaboration with the company CEMKA for economic evaluations, the incidence of AED-related hospitalizations in the medical department of French public hospitals was studied. The evaluation was performed over 14 consecutive days in 62 hospital departments, which were selected randomly. The total number of 3137 patients were hospitalized for a mean duration of 9 days and they were using a mean number of six different drugs. Taking into account the number of about 4 million patients admitted per year in the hospitals represented, it was estimated that the total number of AED-related hospitalizations amounts to about 130,000 annually (CI95%: 100,916-156,620). Using established cost calculations for hospitalized days (AP-HP, results of 1996) the mean cost for an AED-related hospitalization was estimated to be about FF16,000.