Objective: To determine whether passive as well as active smoking by women or smoking by men is associated with delayed conception, after adjustment for confounding factors. Design: Population study of couples expecting a baby. Logistic regression was performed to identify factors associated with delayed conception. Setting: The Avon Health Authority area, United Kingdom. Patient(s): All couples expected to deliver between April 1991 and December 1992. Intervention(s): Questionnaires administered early in pregnancy. Main Outcome Measure(s): Time taken to conceive, categorized as 3 years. Result(s): After correction for confounding factors, delayed conception was statistically significantly associated with both active smoking by the woman (odds ratio [OR] 1.23 [95% CI 0.98–1.49] for > 6 months and 1.54 [95% CI 1.19–2.01] for >12 months) and her exposure to passive smoking (OR 1.17 [95% CI 1.02–1.37] and 1.14 [95% CI 0.92–1.42]) compared with women with no exposure to tobacco smoke (referent). Heavy smoking by men was independently associated with delayed conception. In active smokers, the effect increased with the number of cigarettes. Conclusion(s): Smoking by men and passive and active smoking by women are associated with delayed conception.