In a series of 184 patients with IgA glomerulonephritis (IgA GN) 114 (62%) had concomitant diseases compared with 35 (36%) in a control series consisting of 96 patients with mesangial glomerulonephritis of other type than IgA GN. This difference was statistically significant. Many of the diseases found in association with IgA GN were immunologic or systemic in nature, affected mucous membranes of the respiratory or gastrointestinal tract or were such as known to be associated with high serum IgA level. These clinical findings supported the idea that IgA GN is often a manifestation of various systemic disease processes.