Forty tracheas from children with tracheo-oesophageal fistulae were histologically analysed for structural deformity. Thirty (75%) had a deficiency of cartilage; 24 (60%) showed an increase in the length of the transverse muscle, and 26 (65%) had a longer than average internal perimeter. Only six tracheas were entirely normal. The position of the fistula bore little relationship to the abnormalities found. Since in only nine patients were abnormalities restricted to the site of the fistula, we believe that careful examination of the whole length of the trachea is needed at the time of surgery to determine the extent of the anomaly and to anticipate tracheal problems. This is also relevant to autopsy studies and to elucidate the cause of death.