Monitoring preterm births is essential given their impact on infant morbidity and mortality and their economic and social costs. This article is based on data from the Information System on Live Births (SINASC), implemented in 1990 and expanded gradually to cover 90% of all births in the country. Preterm birth time trends are presented for Brazil, regions, and capitals from 1994 to 2005. At the national level, there was an increase in the preterm birth rate, accompanied by a reduction in the proportion of missing information on gestational age. The Southeast, South, and Central-West regions followed the national trend, while the preterm birth rate fell in the North and Northeast regions. We compared the findings from SINASC with those from population-based studies. The coverage and quality of SINASC has increased over time, but problems with the determination of gestational age still remain, leading to underestimation of preterm birth rates. Due to the importance of SINASC for monitoring, further efforts are needed to improve the system's accuracy.