The occurrence and determinants of preterm delivery were studied in two population-based birth cohorts from northern Finland. In the first cohort of 1966 there were 11,475 singleton deliveries and in the later cohort of 1985-1986, 8,888. The overall incidence (percentage) of preterm deliveries fell from 9.1 to 4.8, including a reduction from 8.8 to 3.4 for spontaneous preterm deliveries. For iatrogenic ones, however, there was an increase from 0.3 to 1.4. The relative risks, associated with such known maternal determinants as unmarried status, smoking, low educational level, age over 34 years, unwantedness of the pregnancy, and poor earlier obstetric history, had about the same level in these two series. However, despite a favorable change in the distribution of most of these determinants over the 20 years, only a small part of the decrease of the total incidence of preterm delivery could be attributed to these changes, as a consistent and substantial reduction occurred in the incidence of spontaneous preterm birth in all categories of each single determinant. The increase in the proportion of iatrogenic preterm deliveries was accompanied by these being more common at lower than at higher socioeconomic levels in 1985-1986, whereas the social gradient appeared to be reversed in 1966.