OBJECTIVE:To investigate the extent to which fecundability is associated with active smoking, time since smoking cessation, and passive smoking. DESIGN:Prospective cohort study. SETTING:Denmark, 2007-2011. PATIENT(S):A total of 3,773 female pregnancy planners aged 18-40 years. INTERVENTION(S):None. MAIN OUTCOME MEASURE(S):Self-reported pregnancy. Fecundability ratios (FRs) and 95% confidence intervals (CIs) were estimated using a proportional probabilities model that adjusted for menstrual cycle at risk and potential confounders. RESULT(S):Among current smokers, smoking duration of ≥10 years was associated with reduced fecundability compared with never smokers (FR, 0.85, 95% CI 0.72-1.00). Former smokers who had smoked ≥10 pack-years had reduced fecundability regardless of when they quit smoking (1-1.9 years FR, 0.83, 95% CI 0.54-1.27; ≥2 years FR, 0.73, 95% CI 0.53-1.02). Among never smokers, the FRs were 1.04 (95% CI 0.89-1.21) for passive smoking in early life and 0.92 (95% CI 0.82-1.03) for passive smoking in adulthood. CONCLUSION(S):Among Danish pregnancy planners, cumulative exposure to active cigarette smoking was associated with delayed conception among current and former smokers. Time since smoking cessation and passive smoking were not appreciably associated with fecundability.