OBJECTIVES: Radiation-induced cytogenetic damage in somatic cells has raised concern that low-dose ionising radiation can also damage germ cells and influence gamete production and/or function, resulting in decreased fertility. Time to pregnancy (TTP) was used to investigate whether exposure to gamma-radiation affected fertility among the residents of cobalt-60-contaminated buildings in Taiwan. METHODS: This was a retrospective pregnancy-based study of 357 pregnancies born to 124 exposed couples. Both the cumulative dose and the dose rate for each pregnancy was estimated based on a physical dose reconstruction programme. The comparison population consisted of 612 pregnancies born to 225 couples randomly sampled from the Taiwan general population. Information on TTP was collected by personal interviews. Fecundability ratios (FRs) were calculated with a discrete proportional hazards model. RESULTS: For exposed mothers, fertility decreased significantly when unprotected intercourse began during the period of living in the radiation-contaminated buildings (FR 0.75, 95% CI 0.61 to 0.92). The effect was borderline significant for fathers (FR 0.83, 95% CI 0.68 to 1.02). There was evidence that prolonged TTP was associated with the rate of exposure for both mothers and fathers (tests for trend: female, p=0.0006; male, p=0.03), especially evident for dose rates > or =10 mSv/year (female, FR 0.60, 95% CI 0.43 to 0.84; male, FR 0.68, 95% CI 0.49 to 0.96). CONCLUSIONS: Our findings suggest that exposure to low-dose ionising radiation of cobalt-60-contaminated buildings may decrease fertility, especially in females. Fertility declined with increasing concurrent dose but not with cumulative dose.