Ultrasound (US) was used to determine the congruity of the shoulder in 22 children with a deformity of the shoulder secondary to chronic obstetric brachial plexus palsy. There were 11 boys and 11 girls with a mean age of 4.75 years (0.83 to 13.92). The shoulder was scanned in the axial plane using a posterior approach with the arm internally rotated. The humeral head was classified as being either congruent or incongruent. The US appearance was compared with that on clinical examination and related to the intraoperative findings. All 17 shoulders diagnosed as incongruent on US were found to be incongruent at operation, whereas three diagnosed as congruent by US were found to be incongruent at operation. The diagnostic accuracy of US for the identification of shoulder incongruity was 82% when compared with the findings at surgery. US is a valuable, but not infallible tool, for the detection of incongruity of the shoulder.