The value of prenatal care is controversial and difficult to establish. A national policy for improving perinatal outcomes was proposed and applied throughout Andalusia (Southern Spain) in 1984. Here we report the results of an evaluation of this health care program as regards the prevention of preterm delivery. Effectiveness of prenatal care was assessed on the basis of two case-control studies in a hospital setting: one performed before the program was implemented (1981-1982) and the second one six years after the program began (1990-1993). A total of 229 cases and 395 controls for the period 1981-1982, and 207 cases and 381 controls for 1990-1993 were selected. Prenatal care was assessed based on the number of prenatal care visits, the date of the first visit, and an American composite index adjusting for gestational age. Multiple-factor adjusted odds ratios and their 95% confidence intervals (CI) were estimated using unconditional logistic regression analysis. The use of prenatal care significantly improved across time: the proportion of women receiving no prenatal care decreased from over 30% to less than 5%, and the proportion of women starting prenatal care in the first trimester for 1990-1993 was three times greater than the figure for 1981-1982. In the 1981-1982 case-control study, the date of first visit and the composite index were shown to be unrelated to preterm birth risk; and the number of visits yielded a significant association, although no definite trend could be established. In the 1990-1993 case-control study, a clear and significant relationship was observed between the number of prenatal care visits, the trimester of the first visit, and the adequacy of care according to the composite index. This latter variable, reflecting a more stringent standard of prenatal care, was selected by a stepwise logistic regression analysis as the best predictor for preterm birth risk. The results suggest that the present Andalusian program helps prevent preterm delivery. Nonetheless, its minimum standards should be raised to further decrease preterm birth risk.