Arthritis related to the primary disease was detected in 10 of 74 (13.5% patients seen primarily for leukemia [2 of 12 patients with acute (16.6% and 8 of 62 (12.4%) with chronic leukemia]. To these were added 18 ad ditional patients previously diagnosed of having arthritis of leukemia.Clinical analyses of 28 patients with arthritis of leukemia showed that the arthritis of acute leukemia was commonly asymmetric, additive, polyarticular and often part of the initial manifestation of the disease. The arthritis of chronic leukemia was similar t o the acute but was often symmetric and usually appeared as a late manifestation. X-ray films of affected joints were abnormal in 7 patients. Mild hyperuricemia was found in 15.8%, rheumatoid factor in none and antinuclear antibodies in 11% of patients with arthritis of leukemia. Leukemic infiltration of the synovial membrane was found in all 3 of the patients who underwent synovial biopsy and is believed to be the cause of the arthritis.Although arthritis is a known manifestation of acute leukemia (1-5), its occurrence in chronic leukemia is not well recognized ( 6 , 7 ) , and its prevalence has not been reported. This study describes the clinical characteristics and frequency of joint manifestations in a group of patients with chronic and acute leukemia. The incidence of rheumatoid factor, hyperuricemia, antinuclear antibodies and roentgenogram abnormalities was determined. Further, histologic examinations of synovial biopsies were performed in 3 patients.