To determine whether outcome in chronically constipated and encopretic children with abnormal defecation dynamics could be improved with biofeedback training, we randomly assigned patients, 5 to 16 years of age, to receive conventional treatment alone (n = 19) or conventional plus biofeedback treatment (n = 22) and evaluated physiologic outcome at 7 months and clinical outcome at 7 and 12 months. Eighty-six percent of patients learned normal defecation dynamics with up to six biofeedback sessions. At 7 months, 13% of conventionally treated and 77% of biofeedback-treated patients had normal defecation dynamics (p less than 0.01); one conventionally treated (5%) and 12 biofeedback-treated patients (55%) had recovered (p less than 0.01). Learning normal defecation dynamics was correlated with clinical recovery (p less than 0.01). At 7 months, 11% of patients with normal defecation dynamics after biofeedback treatment had abnormal defecation dynamics, and 71% of the biofeedback-treated patients with normal defecation dynamics recovered. At 12 months, 16% of conventionally treated and 50% of biofeedback-treated patients had recovered (p less than 0.05). Balloon defecation did not improve significantly in those who learned normal defecation dynamics. Therefore the ability to defecate balloons is apparently not dependent on the normal function of the external and sphincter and pelvic floor muscles alone. Biofeedback treatment is complementary to a good conventional therapeutic regimen in patients with abnormal defecation dynamics.