Eighteen children with idiopathic restrictive cardiomyopathy (IRC) were studied in an attempt to identify potential predictors of poor outcome. Four patients presented with low cardiac output (CO) syndrome. Fourteen remaining patients were minimally symptomatic at presentation but developed a low CO syndrome at a mean of 2.8 +/- 2.3 years after diagnosis. At the time of development of low CO in the 18 patients, mean left ventricular end-diastolic pressure was 27 mm Hg, right ventricular end-diastolic pressure was 18 mm Hg, cardiac index was 2.5 L/min/m(2), and pulmonary vascular resistance index (PVRI) was 8.8 U-m(2). Eleven of the 18 patients underwent cardiac transplantation. One died perioperatively from donor right-sided cardiac failure and 10 survived. Six were not transplanted and died, including 3 in whom transplantation was precluded secondary to extremely elevated PVRI. One patient is alive with right-sided cardiac failure. Ten of our 18 patients had pulmonary hypertension (PVRI >6 U-m(2)) at the time of referral for cardiac transplant and/or development of low CO syndrome. In comparison, children with dilated cardiomyopathy who were referred for heart transplant during the same time period had a PVRI that was significantly lower (5.2 U-m(2)). Elevated PVRI was associated with death (p <0.01) and 40% of our children with pulmonary hypertension were precluded from receiving an orthotopic heart transplant because their pulmonary hypertension was so severe. No risk factors for the development of pulmonary hypertension were identified; therefore, all children with IRC should undergo serial monitoring of their PVRI, and any increase should prompt a transplant evaluation.