Bloom's syndrome is a rare autosomal recessive disease. All patients with Bloom's syndrome have prenatally onset growth retardation and an increased tendency to develop various types of cancer. Features other than these are not constant and may not be present in some of the patients. Reason for the phenotypic heterogeneity is not clear. Different mutations in the same locus may explain the heterogeneous phenotypes in different ethnic groups. Here we present a seven-year-old boy and his four-year-old sister, both with Bloom's syndrome, who exhibit different clinical features with respect to sun-sensitive skin lesions. The sister has severe facial sun-sensitive skin lesions whereas her brother has none. It is expected that two siblings who are supposed to have the same mutation should also have similar clinical features. Possible role of environmental effects and sex are discussed.