Objectives: The aim of this study was to establish the socioepidemiologic characteristics of the victims of sudden infant death syndrome and to calculate its prevalence during the first year of life. Methods: Observational, retrospective, descriptive study based on the review of autopsy protocols and questionnaires applied to families that had cases of infants death during 1997 and 1998 in Porto Alegre. Results: From 335 infants that died, 21 were included in SIDS criteria although the death certificate did not register this cause. The infants had no previous apnea or any acute disease that could justify the decease. Death occurred predominantly in cold months (winter and autumn) and none of the infants were usually put to sleep in supine position. Families had low income (3 minimum salaries / month). 71% of the mothers were smokers and 42% used alcohol during pregnancy. SIDS prevalence was estimated as 6.3% from the total cases of decease, and the specific mortality coefficient for SIDS was 4.5:10,000 live births. Conclusions: The infant’s risk profile identified resembles very much that of the international literature. However as SIDS is misdiagnosed in our community, it has not been included in the official rates of infant mortality. As a consequence, no risk reducing campaigns have ever been established.