Renal function was studied in 25 patients with IgA nephropathy, aged 15--48 years, during a mean follow-up period of 47 months. GFR remained normal in ten patients but decreased in 15, necessitating regular hemodialysis in five. The evolution of GFR was not related to the degree of increase of the serum IgA level or to the presence of recurrent respiratory tract infections. The mean age of the patients with decreased GFR was ten years older and the mean follow-up time 20 months longer than of patients with maintained renal function. Also these patients had more proteinuria and were more frequently hypertensive. Their initial renal biopsies showed histologic evidence of nephron loss. The hypothesis is discussed that IgA nephropathy in adults not infrequently follows a slowly progressive course.