The formation of specific IgE, IgG1, and IgG4 antibodies was investigated by immunoblotting during hyposensitization with timothy grass-pollen extract and 6 years later, Until the end of immunotherapy, specific IgG antibody levels increased. Also simultaneously, the number of allergenic components detected by IgG increased. However, this IgG response was similar in responding and nonresponding patients; thus, it did not correlate with the clinical outcome of the therapy. More allergenic compounds were also detected by IgE on immunoblots, but again without correlation to success of therapy. Six years after immunotherapy, the therapeutic effect was still present, although by now the observed immunoglobulin-binding patterns were similar to patterns observed in the same patients' sera collected before the initiation of hyposensitization. Thus, changes of antibody-binding patterns in immunoblot do not relate to the success or failure of immunotherapy.