One hundred fifty-seven children, aged 2 days to 15 years, had rectal biopsies for suspected Hirschsprung's disease from 1980 to 1987. Sixty had adequate suction rectal biopsies stained for acetylcholinesterase (ACE); 41 of these were reported as normal and eight showed atypical innervation in children who progressed satisfactorily without surgery. Two showed hyperganglionosis and nine were from children with Hirschsprung's disease. Four of the latter nine biopsies (44%) gave false-negative results in infants aged 9, 10, 13 days, and 6 weeks. The last had total colonic aganglionosis. There were two false-positive results, one in a normal child with atypical innervation and the other in a child with hyperganglionosis. Thus, six of 60 (10%) of ACE-stained suction rectal biopsies gave misleading results.