An unmatched case control study of molar pregnancy was carried out at this hospital between 1978 and 1987 to investigate the influence of maternal age and ethnic group on the incidence of complete and partial hydatidiform mole. The age specific incidence of complete mole was minimal between the ages of 30 and 34 years (relative risk 1), showed a minor peak in teenagers (relative risk 3.1, 95% confidence interval 6.5-1.4), and a major peak in those of 35 years and over. Between 35 and 39 years the relative risk was 2.5 (95% CI 6.2-1.0) and at 40 years or more the relative risk was 9.8 (95% CI 28.9-3.3). No age group showed a significantly increased risk of partial mole. The women of Abu Dhabi had increased risks of both forms of molar pregnancy relative to women in Nottingham, England (relative risk 1): the risk of complete mole was increased threefold (95% CI 4.2-2.2) and that of partial mole twofold (95% CI 4.0-1.2). The increased risk of complete mole was greatest in Gulf Arabs (mainly Omanis and Yemenis) who had a sixfold increase in crude relative risk (95% CI 10.7-3.5). The increased risks of complete mole associated with maternal ethnic group remained after adjustment for maternal age distribution.