The limited knowledge of the cellular mediators of renal scarring hampers progress in the management of progressive chronic renal failure (CRF). We have studied 38 patients with biopsy-proven mesangial IgA nephropathy with emphasis on attempting to define the role of myofibroblasts (alpha-smooth muscle actin/SMA-positive cells) in renal scarring. In 18 untreated patients, correlations were undertaken between known histological parameters of progression as well as the presence of myofibroblasts in tissues and the clinical outcome. alpha-SMA staining by an avidin-biotin-peroxidase method was confined to a large extent to the vascular smooth muscle cells of normal kidneys but extended to the tubulointerstitium and periglomerular space in scarred kidneys. Mild glomerular staining was also noted. The interstitial immunostain followed a similar distribution to that of interstitial type III collagen. Morphometric analysis showed the interstitial alpha-SMA staining to be a reliable histological predictor of outcome as it discriminated between progressors and non-progressors (chi 2 = 4.923, P = 0.026). The intensity of the interstitial alpha-SMA staining correlated with renal functional outcome; inversely with the reciprocal of serum creatinine slopes (r = -0.466, P < 0.025) and positively with the serum creatinine value at the end of the observation period (r = 0.704, P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)