This paper reports on the response rate and completeness of item response in a self-administered postal survey questionnaire on use of medication by retirees 60 years or older under the Brazilian Social Security System, in Belo Horizonte, Minas Gerais State, in 2003. Questionnaires were sent in two rounds to 800 postal addresses of subjects selected by simple random sampling. The response rate was 47.8%, and there were no significant differences in the selected characteristics between respondents and non-respondents, or between early and late respondents. For almost all socio-demographic and health variables, item omission was less than or equal to 5% for both the entire sample and early or late responders. For all the medicines used, the dose and laboratory were the main items omitted. The findings indicate that details on drug use should be reevaluated in self-administered questionnaires.