We report an intestinal pseudo-obstruction syndrome occurring in a 6-month-old infant girl suffering from acquired major and persistent abdominal distension, which 2 months later required ileostomy. Histologic examination of samples of the small intestine showed considerable inflammatory reaction in the muscular layers of the intestinal wall. Steroid therapy, begun as soon as the histologic results were known, resulted in recovery. In a similar case, reported previously, steroid therapy used after the lesions had already caused fibrosis and atrophy of the intestinal wall, proved ineffective. This particular form of pseudo-obstruction is classified as "idiopathic myositis of the small intestine." It is important to identify the condition because an early course of steroid therapy, before the appearance of fibrotic lesions, could improve the prognosis.