The present study comprises 173 breast cancers in women living in Sweden. Mastectomy was performed and the surgical specimens were thoroughly scrutinized histopathologically with special attention given to the mammary tissue outside the dominant mass. Fifty-two patients (30 percent) had multifocal growth in the same breast as the dominant breast cancer. The patients have been followed up for 3 years after operation and compared with age-matched controls. The multifocal growth was not correlated with age, size of the tumor or death of the patient within 3 years. A previous diagnosis of benign breast disease was significantly more common among the cancer patients than among the controls, although it was unrelated to multifocal growth. This study stresses the importance of considering the high incidence of multifocal growth of breast cancer when discussing treatment by operation less radical than mastectomy.