Recently etretinate was demonstrated in the blood of women on acitretin. Because of the proven teratogenic effects of etretinate we tried to trace all users of acitretin in the Netherlands via all drug dispensing outlets (1450 community pharmacies, 95 hospital pharmacies and 636 general practitioners). A request for information as regards date of birth, sex, type of prescriber, and number of dispensed capsules was followed by a response rate of 87%. In 61% of all responding outlets a prescription of acitretin had been filled. Of these 1153 outlets 40% had dispensed the drug to one user, whereas 52% had dispensed acitretin to 2-5 patients. The 2717 patients consisted of 1500 men (55%) and 1217 women with mean ages of 48 and 53 years, respectively. Of the women 31% were in the age-group of 15-45 years and of the male users, 45% (p less than 0.0001), whereas in other age groups there was no difference. In 80% of cases acitretin had been prescribed by dermatologists. This study demonstrated that it is possible to trace a cohort of users quickly and almost completely. As most large-scale problems with adverse reactions are discovered in the first 2 years of marketing when exposure and morbidity registries are still collecting information, ad hoc cohort formation may be a useful resource for postmarketing surveillance.