The possible relation between Na,K-pump concentration and left ventricular (LV) function was studied in 24 patients with suspected idiopathic dilated cardiomyopathy. This was done by measurement of 3H-ouabain binding to biopsies obtained during left-sided heart catheterization. In all patients light microscopy of biopsies was compatible with dilated cardiomyopathy. Nineteen patients had impaired LV function as defined by NYHA/WHO and a Na,K-pump concentration of 331 ± 19 pmol/g wet weight, wheras 5 patients had normal LV function and a Na,K-pump concentration of 559 ± 62 pmol/ g wet weight (p < 0.001). The correlation between Na,K-pump concentration and ejection fraction was highly significant (n = 24, r = 0.81, p < 0.001). There was no correlation between volume fraction of collagen tissue and Na,K-pump concentration in the biopsies (n = 24, r = -0.08, p < 0.80), indicating that the decrease in Na,K-pump concentration with dilated cardiomyopathy is not the simple outcome of increased fibrosis in the myocardium. The results indicate that the decrease in Na,K-pump concentration may be of importance for myocardial dysfunction and suggest a simple biochemical assessment of dilated cardiomyopathy by measurement of 3H-ouabain binding.