Leukemia was more common among first-degree relatives of patients with chronic lymphocytic leukemia than could be expected from the incidence of the disease in the population of New Zealand. This finding confirms earlier results which showed a similar, though smaller, increase in leukemia incidence in all classes of relatives of patients with all forms of leukemia. Laboratory studies indicated a diminished serum γ-globulin concentration and an unusual serum haptoglobin distribution in the leukemia patients. The familial aggregations appear to have been determined at least partly by genetic factors, but the precise mechanism by which they were produced remains to be determined.