Three children with congenital constriction band syndrome affecting their upper extremities demonstrated clinical and electrophysiologic signs of a complete ulnar nerve palsy. Two of the children were diagnosed immediately postpartum with the subtle findings of an intrinsic minus posture of their hand and inability to actively extend their fingers at the proximal interphalangeal joints. One child had at least 5.5 months of intrauterine compression of the ulnar nerve detected by ultrasound examination at 18 weeks. Despite early release of the constriction bands, at 3 months in 2 children and at 6 months in 1 child, the ulnar nerve palsies persisted for a mean follow-up period of 7 years. If clinical examination of an infant with constriction band syndrome is indicative of a complete ulnar nerve palsy, the constriction band should be released as early as possible. If surgical exploration reveals significant compression of the ulnar nerve, consideration should be given to excising the involved segment of nerve with immediate primary nerve repair or nerve grafting because even early release of the constriction band does not seem to result in neurologic improvement in long-term follow-up studies.