This work was undertaken to examine the immunoregulation of T lymphocytes in patients with IgA nephropathy. Fifty patients and thirty-seven control subjects were studied in an infection-free interval. T lymphocyte subpopulations were determined using OKT monoclonal antibodies against helper (OKT4) and suppressor (OKT8) T cell subsets. The proportions of T lymphocyte subpopulations did not differ between patients and controls. Patients with significant renal impairment demonstrated a reduced OKT4/T8 ratio (p less than 0.001) due to an absolute reduction of T helper cells (p less than 0.02) and an increase of T suppressor cells (p less than 0.001). Longitudinal studies performed in 13 patients revealed consistent findings during clinical quiescence. However, synpharyngitic macroscopic haematuria was associated with a rise in T4 positive cells and a simultaneous reduction of T8 positive cells; these changes reverted to pre-infection values when the infection subsided. Functional studies assessing T lymphocyte activities including in vitro immunoglobulin synthesis by cultured peripheral blood mononuclear cells, thymidine uptake by cultured lymphocytes and T lymphocytes activation with expression of interleukin-2 receptors were measured in unstimulated and mitogen stimulated cultures. No significant difference between patients with IgA nephropathy in clinical quiescence and the control subjects was demonstrated. Our results failed to support a shift in the immuno-regulatory T lymphocyte subpopulations during clinical quiescence but a more profound defect in immunoregulation may probably occur during clinical exacerbation.