In 34 consecutive patients in whom the diagnosis of Hirschsprung's disease (HD) was suspected, the reliability of clinical symptoms, radiological parameters, and anorectal manometry was retrospectively and blindly evaluated by three independent investigators. In 19 patients the diagnosis of HD was histologically proved, while in the remaining 15 cases the diagnosis of idiopathic constipation was justified by persistent success of medical treatment at regular follow-up. Anorectal manometry was correctly diagnostic in all the patients who were examined. The recto and inhibitory reflex, recorded in all the patients with idiopathic constipation was absent in patients suffering from HD. Moreover, other manometric parameters, peculiar to HD were found: a significant lower anal resting pressure and pain threshold, as well as a decreased frequency of spontaneous rhythmic oscillations. The reliability of radiological and clinical data for these diagnoses appeared to be inadequate. Therefore, the diagnosis of HD can be confidently stated only on the basis of manometric investigations.