A method was developed for monitoring industrial workers and others exposed to environmental agents which may impair fertility. National birth probabilities specific for maternal birth cohort, age, parity and race were used to derive expected fertility. Observed fertility was obtained by questionnaire. Standardized fertility ratios were computed for exposure and non-exposure periods and compared. Analytic techniques were validated by applying the method to a group of 36 male factory employees working in an agricultural chemical division (ACD) where pesticides including the nematocide dibromochloropropane were formulated. Employees (12) in mid-1977 had severely depressed sperm counts related to occupational exposure. Standardized fertility ratio (SFR) computed from data available in mid-1977 for the period at risk from employment in the ACD (SFR = 0.75) was significantly lower than those derived for the entire not-at-risk period (SFR = 1.88) and the portion related to employment in other areas of the factory (SFR = 2.16). Similar differences were evident from data available several years earlier, demonstrating that the surveillance technique would have been capable of detecting occupationally induced infertility among these workers in advance of the actual discovery date.