Over a 10-year period sera of 4000 pediatric patients were subjected to agar gel electrophoresis and immunoelectrophoresis. Retrospective examination of the electrophoresis patterns indicated that single or multiple homogeneous immunoglobulin components were present in sera of 155 children (3.9%). They were most frequently found in patients suffering from primary and secondary immunodeficiency diseases, hematological malignancies, autoimmune diseases, and severe aplastic anemia. Follow-up analysis revealed that most of these monoclonal gammopathies were transient. The monoclonal gammopathies in the serum of 79 patients were identified by immunoblotting for class and light-chain isotypes. A marked absence of IgA monoclonal gammopathies and a predominance of monoclonal gammopathies of the lambda light-chain isotype were found. Most of the B-cell mono- or oligoclonal proliferations in children can probably develop due to a disturbance in the regulatory T-cell function.