The current update of the US man-made vitreous fiber production worker cohort includes women for the first time. Preliminary comparisons of 3,820 female and 27,767 male workers from 11 participating fibrous glass plants show different hiring patterns during World War II. The gender-specific person-year distributions are similar with respect to duration of employment and time since first employment. The current follow-up of 118,559 person-years for women provides an estimated 80% power to detect a threefold relative risk of respiratory cancer for women who worked more than 14 years in these plants, based on a Poisson regression analysis of the cohort rates. When women comprise a small fraction of the cohort, the statistical power may be inadequate to detect risks of the magnitude typically of interest in studies of men.