Infantile hypertrophic pyloric stenosis (IHPS) is characterized by hypertrophy of the pyloric muscle. It is not clearly understood whether pyloric muscle enlargement is due to hypertrophy (increase in cell size) or hyperplasia (increase in cell number). In the present study, we investigated proliferative activity as well as size and number of smooth muscle cells to understand the mechanism of pyloric muscle enlargement in IHPS. Full thickness muscle biopsy specimens were obtained from 18 IHPS patients at pyloromyotomy and from 11 age-matched controls. Formalin-fixed paraffin sections were immunostained with MAb MIB-1, which stains cells in the proliferating phase of the cell cycle. The proliferative index (PI) was calculated as the percentage of positive cell nuclei. Smooth muscle cell number per bundle and cell size were measured with an image analyzer. The mean PI in IHPS (9.6 +/- 5.7%) was significantly higher than that of controls (1.3 +/- 1.2%) (p < 0.01). There was a significant inverse correlation between PI and age at operation. Smooth muscle cell number per bundle in IHPS (240.6 +/- 129.4) was significantly greater than that of the controls (134.1 +/- 49.8) (p < 0.05). Smooth muscle cell size in IHPS (298.5 +/- 59.0 microm2) was also significantly greater than that of controls (154.3 +/- 21.5 microm2) (p < 0.01). Our findings suggest that hypertrophy-and hyperplasia as well-play important roles in increasing pyloric smooth muscle mass in IHPS.