Two boys from different families had primary carnitine deficiency: one had cardiomyopathy and myopathy, and the other had hypoglycemia and myopathy but no cardiomyopathy. Uptake of carnitine by cultured fibroblasts was negligible in both patients. Vmax for carnitine transport was reduced to 50% of controls' value in the parents and one brother (who had hypertrophic cardiomyopathy) of the first patient. A brother of the second non-cardiopathic patient died at an early age with autopsy findings of a dilated cardiomyopathy and low cardiac carnitine. Autosomal recessive primary carnitine deficiency can express a variable phenotype in different families as well as within the same family. Heterozygotes can manifest heart involvement.