Absence of ganglion cells in rectosigmoid and rectum; usu. presents as complete colonic obstruction in infants (failure to pass meconium in 1st 24 hours) or chronic constipation in children. Male-to-female ratio is 4 to 1. Definitive diagnosis is rectal biopsy. Dilated segment is proximal to aganglionic area. Surgical treatment is Duhamel, Swenson, Soave, or Boley procedures.