To characterize the phenotypic alteration in mesangial cells in human glomerulonephritis, we investigated the expression of nonmuscle-type myosin heavy chain, SMemb, and α-smooth muscle actin (α-SM actin) in IgA nephropathy. The expression of SMemb and α-SM actin was examined by immunohistochemistry in biopsy specimens from 45 patients with IgA nephropathy. We examined a total of 489 glomeruli representing all patients enrolled, and found that mesangial expression of SMemb and α-SM actin was associated with mesangial proliferation. Only mesangial expression of SMemb showed a significant relationship with mesangial matrix accumulation. Semiquantitative evaluation using composite expression scores showed that the expression of SMemb was elevated in the patients with poor renal prognosis. The expression of α-SM actin showed no significant relationship with renal prognosis. These results suggest that mesangial expression of SMemb is an important factor in the progression of IgA nephropathy, and that SMemb and α-SM actin are associated with the activation of mesangial cells by different mechanisms.