Data from two population-based studies in four Norwegian counties were used to calculate the crude incidence of mammary tumours, and the age- and breed-specific incidence of mammary tumours in female dogs of three different breeds. The largest study comprised 14401 histologically verified tumour cases from four counties covered by the Norwegian Canine Cancer Register. The registry covers about 25% of the total Norwegian dog population. The second study was a census in Norway that was sent to all owners of the following breeds: boxers, bichon frisé and Bernese mountain dogs, to estimate the age distribution of the female dog population at risk of developing mammary tumours. The crude incidence of malignant mammary tumours in female dogs of any breed was 53.3%. The highest relative risk ratio of mammary tumours was found in boxers, cocker spaniels, English springer spaniels and dachshunds. The mean age of histologically diagnosed mammary tumours was 7.9 years in boxers and 7.8 years in springer spaniels, compared with 8.8 years in all other breeds. In the four Norwegian counties from 1992 to 1997, the population-based incidence rates (for all ages) of malignant mammary tumours per 1000 female dogs per year were 35.47 in boxers, 3.87 in Bernese mountain dogs and 17.69 in bichon frisé. Mammary cancer is the most common tumour in female dogs in Norway, and represents a population of almost entirely reproductively intact females. The age-specific incidence rates for mammary cancer vary considerably among the three breeds that were studied in detail.