A population-based survey of life-style factors and subfecundity (prolonged time to pregnancy) was conducted between 1984 and 1987 in two cities in Denmark. Altogether, 11,888 women filled out a questionnaire in the last trimester of pregnancy (an 86 percent response rate). After exclusion of women who had been treated for infertility or who did not respond to the question on infertility, 10,886 subjects remained. Among nonsmokers, no association was found between subfecundity (defined as a waiting time of 1 year or more from cessation of contraception to achievement of pregnancy) and consumption of hot caffeinated beverages. For women who smoked and also consumed at least 8 cups of coffee per day (or an equivalent amount of tea), a statistically significant association was seen (odds ratio = 1.35, 95% confidence interval 1.02-1.48) for a wait of 1 year or more. Consumption of coffee was closely related to smoking habits and to a number of social factors, such as education and peer group acceptance, which may play a role in subfecundity.