In a study of 523 patients with breast cancer, delay in diagnosis was examined in relation to stage of disease, age and ethnic origin. Localized disease was found in 44% of the patients, regional disease in 40%, and metastatic disease in 9%. Stage was unknown in 7%. The incidence of breast cancer was significantly higher in Occidental Jews (of European origin) than in Oriental Jews (of Asian or North African origin) or in Arabs. The median and mean age were significantly higher in the Occidental Jewish group. Older patients (greater than 70) had more advanced disease and longer delay. Diagnosis was delayed (greater than 6 weeks) in 43% of the patients. Diagnosis was attributable to the patient in 69% of cases and to the physician in 30%. There was no delay in diagnosis for most of the patients with localized disease, whereas there was for most of those at an advanced stage. When there was a delay, it was longer in those with advanced disease. Delay due to physician responsibility was longer than the delay due to patient responsibility. Although the percentage of patients with delayed diagnosis was similar in the three ethnic groups studied, the stage of disease was significantly more advanced in Oriental Jews and Arabs than in Occidental Jews. The possible explanations for these findings are discussed. The importance of patient education aimed at breast self-examination and the role of the physician in preventing delay in diagnosis are stressed.