Infants and toddlers with constipation usually have a history of infrequent, hard and painful bowel movements, often accompanied by screaming and stool-holding maneuvers. Encopresis, the intermittent and involuntary passage of formed-to-liquid feces, is common. Although constipation most often has a functional cause, the history and physical examination should rule out anatomic, endocrinologic, metabolic or neurologic causes. Hirschsprung's disease--aganglionic megacolon--is a congenital cause of severe constipation that occurs in one of every 5,000 births. Treatment consists of fecal disimpaction, laxatives to prevent future impaction, promotion of regular bowel habits and, finally, toilet training. Parents should be reassured that although this disorder is not life-threatening, several months to years of supportive intervention may be required for effective treatment.