It is becoming evident that IgA nephropathy (IgAN) is the most common glomerular disease, and a frequent cause of end-stage renal failure in both white and Asian populations. Its significance as a public health problem is not known since little epidemiologic research is available in most countries. The apparent geographic variations in the percentage of IgAN in kidney biopsy specimens may reflect different clinical policies for diagnostic tests. As a consequence, the frequency of IgAN cannot be accurately extrapolated from these data for any given population. The highest percentages, reported in Singapore and Japan, may be influenced by the systematic screening of urines in both countries. By contrast, IgAN has been detected rarely in blacks either from the United States or from Africa. However, biopsies are performed infrequently in African patients with only microscopic hematuria. Such ethnic differences may suggest a possible role of genetic factors in the etiology of IgAN. As shown recently in France and Italy, antibiotic therapy of streptococcal infections apparently has not influenced the percentage of IgAN in kidney biopsy specimens. These facts and the rarity of the glomerulonephritis in blacks suggest that infections may not be responsible for the etiology of IgAN. The traditional search for causal agents should be approached more vigorously. It will require innovative epidemiologic efforts to understand the mechanisms by which multiple factors (environmental and genetic) acting together influence the risk of disease.