We investigated whether or not an association could be found between mammary tumours and prior clinical use of medroxyprogesterone acetate (MPA) in bitches. A population-based retrospective agematched case-control study was designed based on interviews with the owners of the bitches. The proportion of bitches with diagnosed mammary tumours (group MT+, n = 98) that had received progestin injections was compared with the proportion in a control group without mammary tumours (group MT-, n=98). In the case group 39%, and in the control group 21% of the bitches had been treated with MPA. A significantly higher number of bitches with mammary tumours had been exposed to progestins, compared with the control group without mammary tumours (odds ratio = 2.32, Chisquare=7.01, p=0.008). Bitches treated clinically with low doses of MPA to avoid oestrus were at a greater risk of developing mammary tumours, the majority of which were histologically malignant (91%).