In Ontario, breast cancer accounts for one death in every 27 among females. In 1938-1956 some 40% of all new cases were registered at the Ontario Cancer Foundation's regional clinics. The five-year crude survival rate for 11,393 women was 45.4%, and for 91 men, 36.3%. Survival rates were strongly affected by extent of disease; when this was allowed for, pregnancy and treatment method were also found to influence survival rate. Simple mastectomy with radiotherapy gave results that appeared comparable to those after radical mastectomy, alone or with radiotherapy. There was a 20% improvement in the crude five-year survival rate over the period of the survey. The need for great caution in interpreting these findings is stressed.