Chronic obstructive pulmonary disease (COPD) is a large-scale social and health problem. Because prevalence studies have been performed only in certain areas of Spain, the Spanish Society of Pneumology and Chest Surgery initiated the IBERPOC study with the aim of determining the prevalence of COPD in several parts of the country. IBERPOC is a multicenter, population-based epidemiological study in which subjects have been selected randomly. Seven full-time pneumologists work full-time in seven different areas of Spain. A study of this nature involves a series of difficulties that should be taken into account at the time of planning studies of a similar nature. After 5,827 attempts to reach subjects, a total of 4,967 useful contacts were made, to provide 97.3% of the 5,104 contacts expected. Of those 4,967 useful contacts, 4,035 subjects completed interviews. Among the 932 refusals, 882 (94.6%) agreed to answer a short telephone questionnaire. The field work took 13 months to complete, 30% more time than had been anticipated. The rates at which subjects could not be traced differed from one zone to another, ranging between 5% and 30%; higher rates were attributable to the absence of telephone data in the corresponding census report. In one zone, the dispersion of the census population over a very large area supposed greater difficulty and delay in carrying out the study. In conclusion, the rate of participation and the time required for recruitment can be considered satisfactory. The performance of field work by trained, full-time pneumologists has been essential to the development and quality of the study. Similar epidemiological studies should consider problems derived from the lack of adequate census data, as well as to avoid defining areas of extreme population dispersion in order to facilitate field work.