Clotrimazole is an antimycotic imidazole derivative that interferes with cellular Ca 2+ homeostasis. We investigated the effects of clotrimazole on acute lymphoblastic leukemia (ALL) cells. Treatment with 10 M clotrimazole (a concentration achievable in vivo) reduced cell recovery from cultures of all nine ALL cell lines studied (B-lineage: OP-1, SUP-B15, RS4;11, NALM6, REH, and 380; T-lineage: MOLT4, CCRF-CEM, and CEM-C7). After 4 days of culture, median cell recovery was 10% (range, Ͻ1% to 37%) of cell recovery in parallel untreated cultures. Clotrimazole also inhibited recovery of primary ALL cells cultured on stromal feeder layers. After leukemic cells from 16 cases of ALL were cultured for 7 days with 10 M clotrimazole, median cell recovery was Ͻ1% (range, Ͻ1% to 16%) of that in parallel untreated cultures. Clotrimazole was active against leukemic cells with genetic abnormalities associated with poor response to therapy and against multidrug-resistant cell lines. In contrast, mature T lymphocytes and bone marrow stromal cells were not affected. Clotrimazole induced depletion of intracellular Ca 2+ stores in ALL cells, which was followed by apoptosis, as shown by annexin V binding and DNA fragmentation. Thus, clotrimazole is cytotoxic to ALL cells at concentrations achievable in vivo.