The mean age at natural menopause of 7 328 women participating in a health screening conducted by the Stockholm City and County Council was 50.2 years. The corresponding age encountered for a subgroup of 49 women with hypercalcaemia (very probably due to primary hyperparathyroidism) confirmed in repeated determinations was 45.7 +/- 0.4 (mean +/- S.E.M.) and significantly lower than that (50.1 +/- 0.4 years) of a normocalcaemic age-martched control group (p less than 0.001). The health screening was performed about eight years after the women's menopause. The observation and control groups showed a significant difference with regard to the serum calcium level but no difference was found with respect to marital, parital or socio-economic status or disorders that may result in early or late menopause.