This study was conducted to estimate the frequency of BRCA1 (185delAG) mutation in Egyptian female patients with breast cancer. Forty selected female patients with breast cancer, 80 of their female relatives and 10 healthy females as a control group were included in this study. The age of onset of breast cancer was below 40 years in 25 (62.5%) patients and above 40 years in 15 (37.5%) patients. There were significant differences among the patients regarding the age at menarche before 13 years (p=0.011), onset of breast cancer (p<0.001), parity (p<0.001), first delivery before 30 years of age (p=0.04), breast feeding (p=0.002), and positive family history (p<0.001). The frequency of BRCA1 (185delAG) mutation was 10% in the patients. Eight percent of patients with early onset below 40 years and 13.5% of patients with onset after 40 years were heterozygotes for the mutation. Three percent of patients with unilateral breast cancer, 40% of patients with bilateral breast cancer and 50% of patients with breastovarian cancer were carrying the mutation. Our results indicated that breast-ovarian cancer and bilateral breast cancer patients were likely to have BRCA1 (185delAG) mutation than in unilateral breast cancer. [Turk J Cancer 2008;38(4):167-174]