The diagnosis of twin-twin transfusion syndrome remains a problem. Intravascular infusion of pancuronium bromide, a nondepolarizing neuromuscular blocking agent, for the smaller of a set of twins demonstrated paralysis of both twins in one case. However, in a second case, paralysis was shown only in the smaller twin who demonstrated no transplacental vascular communications postnatally. We suggest that infusion of pancuronium bromide, 'pancuronium test', is a simple and safe method for confirming transplacental communications and aids in the diagnosis of twin-twin transfusion syndrome.