The impact of male age on fecundity remains controversial. Here, a large population study was used to investigate the effect of paternal age on time to conception. All couples in the Avon Health district expecting a baby between 1 April 1991 and 31 December 1992 were eligible. Questionnaires completed by both the man and the woman at 18 weeks gestation covered specific fertility factors, e.g. parity, paternity, cohabitation and oral contraception; and non-specific factors, e.g. educational achievement, housing, cigarette smoking, alcohol consumption, obesity. Logistic regression was used to identify factors independently related to conception in < or =6 or < or =12 months. Of 8515 planned pregnancies, 74% were conceived in < or =6 months, 14% in the second 6 months and 12% after more than a year. Nine variables, including the age of the woman, were independently related to time to conception. After adjustment for these, the likelihood of conception within 6 or 12 months was lower in older men. Compared to men <25 years old, the adjusted odds ratios (95% confidence interval) for conception in < or =12 months were 0.62 (0.40, 0.98), 0.50 (0.31, 0.81) and 0.51 (0.31, 0.86) in men aged 30-34, 35-39 and > or =40 years respectively.