Numerous studies have evaluated T cell subsets and in vitro IgA synthesis in patients with IgA nephropathy. These reports have resulted in the hypothesis that defective regulation of IgA synthesis is important in the pathogenesis of IgA nephropathy. Baseline immunologic measurements were performed in a clinically well characterized group of 19 pediatric and 13 adult (greater than or equal to 18 years) patients with no macrohematuria or intercurrent infection at the time of study. Mean percentages of OKT3 and OKT4 subsets were significantly decreased for the patients as compared to healthy adult controls. Mean T4:T8 ratios were similar for control and patient groups although 7 patients had T4:T8 ratios greater than 2 SD above the control mean. Unstimulated and pokeweed mitogen stimulated in vitro IgA synthesis was similar for patients and controls. During six episodes of macrohematuria in five patients no significant changes occurred for T cell subset percentages, while mean T4:T8 ratios decreased from baseline. Mean serum concentration of IgA increased during these episodes, although in vitro IgA synthesis remained normal. Our data fail to demonstrate a consistent immunoregulatory abnormality for patients with IgA nephropathy.