No previous controlled studies of ovarian germ cell tumours have been reported; however the tumour is similar to germ cell testicular cancer in terms of histology, age-specific incidence rates (i.e. highest rates in young adulthood), and secular trends of increasing incidence. The investigation was designed to determine if maternal hormonal factors which have been found to increase the risk of testis cancer in male offspring are also risk factors for the ovarian tumour. The analysis is based on 73 cases diagnosed before age 35 and 138 age-race matched controls. The cases were identified by tumour registries in Los Angeles (1972-84) and Seattle (1974-84) and controls were selected from friends and/or neighbourhood residents. Interviews were conducted on the telephone with mothers of cases and controls. The primary finding was that mother's use of exogenous hormones (including the hormonal pregnancy test, DES or other supportive hormones, and inadvertant use of oral contraceptives after conception) increased risk (Odds ratio, OR = 3.60, 95% CL = 1.2-13.1). Other maternal factors associated with elevated risk were high pre-pregnancy body mass (OR = 2.7, 95% CL = 1.0-7.6), more rapid achievement of regular menstruation after menarche (OR = 1.8, 95% CL = 0.9-3.8), and age at index pregnancy under 20 (OR = 2.8, 95% CL = 1.0-10.7). In conclusion, these results support findings from testis cancer studies regarding a hormonal aetiology for germ cell tumours, and a mechanism by which oestrogen may affect the germ cells is proposed.