Important changes have taken place in both the economy and health care in Brazil since 1980 and may have affected maternal and child health. Two studies in the city of Pelotas in Southern Brazil provide a unique opportunity for assessing these issues. Cohorts of mothers and infants from 1982 and 1993 were studied from the time of delivery. In both years, all mothers identified in the city's maternity hospitals answered a standardized questionnaire, and their infants were examined. Over 99% of all children born in the city in each of the two years were included in the cohorts, totalling 5,914 live births in 1982 and 5,249 in 1993. Deaths among these children were monitored prospectively, as were all hospital admissions in the 1993 cohort. In 1982, attempts were made to locate a 25% sample of the children at the mean age of 12 months, as well as all of the cohort children at the mean age of 20 months; 82% of the former were located, and thanks to a change in strategy, 87% were traced at 20 months. In 1993, a search was made at 12 months of age for 20% of all children plus all low birthweight infants, and 95% were successfully traced. This paper presents the main methodological aspects of both cohort studies, the principal results of which are presented in the subsequent articles in this supplement.