In a prospective study at the Department of Obstetrics and Gynecology, University Erlangen-Nürnberg, covering the period from January 1st 1971 til December 31st 1973 the incidence of pathological glucose tolerance was examined in 837 women with breast cancer, benign tumours and conditions requiring excision or air cystography and/or mammography because of suspicion of tumour. The glucose tolerance was tested in all patients by oral ingestion with 100 g glucose of i.v. by means of intravenous injection of 0.33 g glucose/kg body weight. The results in 327 women with breast cancer were compared with those in 510 women with benign breast affections. Using matched pairs, the evaluation was done with an electronic data processing system. Diabetogenic factors like age and body weight were thus allowed for. In the total collective 22.3% out of 327 women with breast cancer were manifestly and 6.7% subclinically diabetic. Our findings allow the following conclusions. 1. Manifest diabetes mellitus is found twice as frequently in women with breast cancer compared to women with benign breast affections. 2. In the collective of pairs matched according to age, height and weight (n = 217) 21% of the women with breast cancer have a pathological glucose tolerance, compared with 10% of the women with benign breast affections. 3. Only 25 out of 73 manifestly diabetic women with breast cancer were aware of their metabolic disorder before admission to hospital, whereas 75% of the diabetic women with benign histological findings did know of it.