Twelve of 17 patients (group A) with congestive cardiomyopathy improved hemodynamically during sodium nitroprusside (NP) infusion. Five patients (group B) failed to increase their cardiac output. The two groups were identical in clinical presentation and had comparable cardiomegaly and left ventricular filling pressure (LVFP). However, group A had a baseline cardiac index (CI) lower than 2.5 L/min/m2 and high peripheral systemic (PSR) and total pulmonary vascular resistance (PR). In contrast, group B had a control of CI of higher than 2.5 L/min/m2 and near normal PSR and PR. Furthermore, a highly significant correlation was observed between the calculated levels of control PSR and their subsequent reduction during NP infusion. The higher the initial resistance, the more marked was its fall on NP as documented in group A. In group B, the PSR was probably not high enough for NP to be effective in increasing the cardiac output further by vasodilatation. We conclude that NP infusion may not increase cardiac output in congestive cardiomyopathy, in spite of a high LVFP, if the PSR is near normal.