Of 2075 mammary neoplasms in bitches reported by 14 veterinary schools in the United States and Canada, 1187 were histologically malignant, 557 were benign, and 331 were in the ‘malignancy not determined’ category. There were 12 breeds at significantly high risk for one or more of the major mammary tumour types; 7 of the 12 were hunting breeds. Mixed breeds were at low risk for all tumour types and categories, and collies were at low risk for benign neoplasms. Neutered bitches were only one-third as likely to have neoplasms as were entire bitches. There was no clear difference in distribution of the various tumour types in relation to age at first diagnosis. Among the four geographical regions studied separately, there were more similarities than differences in the rates of mammary neoplasms, especially when adenocarcinomas and malignant mixed mammary tumours were combined for analyses. There was, however, a marked excess of benign neoplasms in the south and west of North America.