The purpose of this study was to investigate the specificity and sensitivity of NADPH-diaphorase (NADPH-D) staining in suction rectal biopsies (SRB) to determine whether it can be used as a diagnostic test for Hirschsprung's disease (HD) and related disorders. We studied SRB material in 80 patients suspected of having such disorders taken at 3, 5, and 7 cm above the pectinate line. Eight-micron sections were stained with hematoxylin and eosin, acetylcholinesterase histochemistry, and NADPH-D histochemistry. Normal biopsy specimens demonstrated strong NADPH-D reactivity in the submucosal ganglia and a large number of NADPH-D-positive fibers in the muscularis mucosae (MM). In contrast, there were no NADPH-D-positive fibers in the MM in patients with HD and hypertrophic nerve trunks stained weakly. Patients with hypoganglionosis (HYPG) demonstrated only a few NADPH-D-positive fibers in the MM and scant submucosal ganglia. Our results show that it is possible to diagnose HD and HYPG in mucosal rectal biopsies containing MM only and stained by NADPH-D histochemistry. As there is no background staining in NADPH-D histochemistry, it is easy to detect NADPH-D-positive fibers. NADPH-D histochemical staining may be an important additional technique for diagnosing HD and related disorders.