The ethnic distribution of 288 patients with chronic lymphocytic leukemia (CLL) from five medical centers in Israel during the decade 1960–1970 was investigated as well as other features of the disease. 80.5% of the patients were of East European descent, 14.5% originated from Asia and Africa and 5% were born in Israel. Based on the ethnic distribution of the elderly population of Israel during the above period, it is suggested that CLL occurs more frequently in European-born Jews than in Asian and African Jews. A low occurrence of Coombs positive hemolytic anemia was revealed, amounting to 7.2% of the patients. Of 68 patients who underwent serum immunoglobulin studies, up to 74% exhibited deficiency of at least one type of immunoglobulin. Low levels of IgA were encountered in 61.2%, IgM in 51.5% and IgG in 29.5% of the patients. 26 of 206 patients with CLL (12.5%), all of East European origin, had an additional primary malignant tumor, cancer of the skin and breast being the most frequent associated malignancies. In 6 patients of the 26, two additional neoplasms were diagnosed. Hypogammaglobulinemia was more frequent in patients with additional primary malignant tumors and in patients with advanced disease. The possible causes for the ethnic distribution of CLL in Israel as well as of other tumors are discussed.