An initial investigation of 313 patients suggested that the ultrasound appearances of gall bladder shape and wall structure might be abnormal in infants with biliary atresia. These observations were then tested in a prospective study. In this study, 346 infants, aged 12 weeks or less, presenting with conjugated hyperbilirubinaemia underwent ultrasound scanning. The gall bladder was identified in 331, 60 of whom had biliary atresia. 11 of the 15 patients in whom the gall bladder was not seen had biliary atresia. Gall bladder shape was normal in 272 patients, 16 of whom had biliary atresia, and irregular in 59, 44 of whom had biliary atresia. The gall bladder wall was regular in 262 patients, 8 of whom had biliary atresia, and irregular in 69, 52 of whom had biliary atresia. Combining these findings gave a sensitivity of 90%, a specificity of 92.4%, a positive predictive value of 75.3%, a negative predictive value of 97.3% and an accuracy rate of 91.9% for ultrasound of the gall bladder in identification of biliary atresia.