The pulmonary vascular bed was evaluated following a Blalock-Taussig shunt performed in 36 patients for decreased pulmonary blood flow. The time of follow-up ranged from one to 21 yr after operation. No patients developed severe pulmonary changes in less than 8 yr after institution of the shunt. Ten of 20 patients having a shunt 8 yr or longer developed some degree of pulmonary vascular disease, mainly in the form of intimal fibrosis. This was severe in 6 of the 20 patients. As the duration of the shunt increased, so did the incidence of pulmonary vascular disease.