The behavior of T lymphocyte subsets was studied in 39 Italian patients with nephrotic syndrome due to idiopathic membranous nephropathy. They took part in a long-term prospective and randomized therapeutic trial based on the 6-month administration of methylprednisolone and chlorambucil. The lymphocyte subsets were evaluated by monoclonal antibodies at the beginning of the trial and at the end of the follow-up period in 23 treated and in 16 untreated patients. Our data seem to suggest that a higher helper-inducer/suppressor-cytotoxic cell ratio before therapy may be a good prognostic index of improved proteinuria. Moreover, the therapeutic schedule does not seem to induce a long-lasting abnormality in cellular immunity.