Colabeling with complement compont C1q or immunoglobulin M (IgM) is occasionally reported in biopsy specimens from patients with IgA nephropathy. The significance of this finding has been questioned. In 83 children and young adults with otherwise typical IgA nephropathy, 15 patients had more than trace mesangial labeling for IgM or C1q. Of these, 14 patients (93%) had greater than 1 + proteinuria at the time of biopsy. This was in marked distinction to the patients with no mesangial labeling for these reactants, only 15% of whom had greater than 1 + proteinuria. The children with IgM or C1q colabel did not differ from those lacking this finding in age at presentation, length of follow-up, or current renal function. In childhood IgA nephropathy, colabeling with IgM or C1q is seen frequently, probably is a function of heavy proteinuria at the time of biopsy, and does not contribute adversely to outcome.