Measurements of plasma anaphylatoxins C3a and C4a were carried out as an indicator of in vivo complement activation in 46 patients suffering from IgA nephropathy/Henoch-Schönlein nephritis. There was a significant correlation between plasma levels of C4a and plasma creatinine and urea. We also found a significant correlation of plasma levels of C3a with plasma creatinine. We propose that the measurement of anaphylatoxin levels provides a sensitive indicator of in vivo complement activation and may serve as an additional method for monitoring the progress of disease in these patients