"Smoldering acute leukemia", a variant of acute myelogenous leukemia, has been recognized with frequent incidence in recent years. This is chracterized by benign clinical course, poor physical findings, leukopenia and mild anemia in the peripheral blood, and apparent infiltration of abnormal myeloblasts in the bone marrow. Immunological studies of the host defence mechanism were made, because the pathogenesis of its "smoldering" course has never been well understood. Nine cases, seen during last 2 years, were investigated for immunological profile, especially the cellular immunity. Purified protein derivative (PPD) skin test, i.e., tuberculin test, was found to be positive in 8 of 9 cases (88.9%). Dinitrochlorobenzene (DNCB) sensitization test showed to be positive in 4 of 6 cases examined (66.9%). Peripheral lymphocyte balstogenesis by stimulating with phytohemagglutinin (PHA) was evaluated using the smear counting method. The blastoid lymphocyte ratio was 55% at the median value (range: 31-68%), compared with 63% in normal young control (age: 25-32) and 41% in normal aged control (age: 60-75). In this report, the cellular immunity in smoldering acute leukemia was proved to be preserved at the normal level and to be more competent than that in aged group. The preserved cellular immunity is considered to explain the phenomenon of "smoldering", in other words, the exacerbating proliferation of leukemic cells is suppressed by immuno-surveillance system.