Fourteen cases of mesangial IgG glomerulonephritis characterized by exclusive or predominant mesangial IgG deposits are reported. The median age at onset was 19 yr (range, 13 to 47 yr). No patient exhibited evidence of systemic lupus erythematous or other systemic diseases. Proteinuria was present in all cases (median, 2.4 g/d; range, 1 to 13 g/d), microscopic hematuria in 12 cases, and macroscopic hematuria in two cases. Five patients were hypertensive at the time of referral. In all cases, renal biopsies revealed mesangial IgG deposits and varying degrees of mesangial matrix expansion, in the absence of significant mesangial cell proliferation. Complement component (mainly C3) deposits were present in virtually all cases. Subepithelial deposits were also noted in nine cases. IgG deposits were polyclonal and consisted mainly of IgG1 and IgG3 subclasses. In electron-microscopic analyses, deposits were electron dense and granular. Treatment was purely supportive. After a mean follow-up period of 11 yr, seven patients had experienced progression to chronic renal failure, including four who had reached end-stage renal failure. Three patients exhibited persistently normal renal function. For one patient, a symptomatic recurrence of mesangial IgG deposits in the renal graft was diagnosed 4 yr after renal transplantation. Such a recurrence highlights the specificity of this type of glomerulonephritis. Mesangial IgG glomerulonephritis is a distinct, albeit rare, type of glomerulonephritis that exhibits far from benign outcome and may recur in renal transplants.