With greatly increased survival rates after childhood leukemia during the last 3 decades, the long-term effects of the treatment have become more evident. The disease and its treatment impair the immune system, but the duration of this impairment is unknown. The authors studied the serum concentrations of immunoglobulins and IgG subclasses in 20 Icelandic children cured of leukemia on average 8 years and 3 months after their treatment ended. Although no marked deviations were found in the concentrations of the main immunoglobulin classes IgA, IgM, IgG, and IgE, the IgG subclass levels were below reference values. The patients had on average 0.9 of age standardized reference values of IgG1, 0.5 of IgG2, 0.8 of IgG3, and 0.7 of IgG4. However, none had any autoimmune diseases or a markedly increased tendency for infections. The results indicate that although the immunoglobulin classes regain their normal values within a few years after cessation of treatment, recovery of the IgG subclasses, especially IgG2, is impaired.