We describe the clinical and immunologic manifestations of a rare genetic disorder, Bloom's syndrome, in 2 sisters. M-va G., 14y.o., had a normal birth weight (3300g), length (50 cm) and growth until the age of 12. She suffered from relatively frequent, but uncomplicated respiratory tract infections. At 12 a brown nevus appeared, that was removed by surgery, at 12y8mo 2nd grade nasopharyngeal carcinoma was removed and the patient was treated with local irradiation and chemotherapy. On admission she presented with characteristic facies, height and weight about the 5th percentile, small head (51 cm), «caf/t au lait» spots, hypoplasia of tonsils, normal menstrual cycle, suppurative sinusitis. Laboratory tests showed leucopenia(2,8-3,3×103 mm3), lymphocytopenia (< 1000 c/mm3), low serum IgA = 0.69, IgM = 0.36 and IgG = 4.83 (g/l), low T-cell numbers: CD3 = 36, CD4 = 20, CD8 = 20, elevated CD16 = 49 and normal CD19 = 12 (%), decreased response to PHA, ConA and PWM. Chromosomal aberrations were found in 10% of cells and anticipated separation of centromers in 14% cells. The sister, 23 y.o, was born with low weight (2500g) and height (49cm). She weighted 7000g at age lyr and marked growth retardation was noted after the age of 12. At presentation she was on the 5th percentile for 18 ys. She did not develop malignancy, but had otherwise similar manifestations to those of her sister. The main difference consisted in the degree of immunodeficiency: her immunologic parameters were lowered, but the relative and absolute numbers of cells were higher than those of the sisters. We had evidence of some clinical and immunologic variability of Bloom's syndrome within one family. The sibling with milder clinical course showed better immune parameters with similar chromosomal changes.