The value of routine bone marrow examination (RBME) in children during and after treatment for standard risk acute lymphoblastic leukemia (SR-ALL) was investigated. The clinical symptoms and peripheral blood findings at the time of bone marrow relapse of 28 children were reviewed and compared with those of 28 matched controls in continuous complete remission. Five (45%) children with bone marrow relapse during maintenance therapy and six (35%) after cessation of cytostatic treatment were asymptomatic at the time of relapse. Signs indicative of relapse during treatment were lymphoblast cells in the peripheral blood, thrombocytopenia, hepatomegaly, anemia, or leukopenia in decreasing order of frequency. After cessation of treatment these signs were lymphoblasts in the peripheral blood, hepatomegaly, splenomegaly, thrombocytopenia, or leukocytosis. Except for one case with thrombocytopenia, no signs suspicious for relapse were found in the control groups. When each sign was evaluated separately only the presence of lymphoblasts in peripheral blood and hepatomegaly were significant symptoms for relapse after cessation of treatment. The mean percentage of lymphoblasts in the bone marrow at the time of relapse was significantly lower for patients with an unpredicted relapse (46.8%) than patients with clinical and/or laboratory evidence of relapse (79.5%). When lymphoblasts were present in the peripheral blood the percentage of lymphoblasts in the bone marrow was always more than 40%, both during and after cessation of treatment. These data suggest a relation between clinical and laboratory symptoms and progression of the disease. It is concluded that 46% of relapses are detected by RBME in the absence of clinical or laboratory symptoms. This early detection may have a positive prognostic influence with more effective treatment for relapsed ALL.