Infantile Pompe's disease is a glycogen storage disorder. Untreated it is lethal within the first year of life. Initial clinical trials with recombinant human acid alpha-glucosidase (rhGAA) have shown enzyme replacement therapy to improve cardiac and skeletal muscle function. B-type natriuretic peptide (BNP) is a neurohormone released by cardiac cells and increasingly used for monitoring heart failure in adults. We report on two infants affected by infantile Pompe's disease and treated with rhGAA, in whom cardiac function was supervised by BNP determination during the first 52 and 26 weeks of life, respectively. In the first patient, BNP (normal < 50 ng/l) increased from 475 (week 4) to 2417 ng/l (week 13) before, and declined continuously from 2696 (week 18) to 107 (week 52) after initiation of rhGAA-treatment. BNP-values reflected improvement of cardiac function earlier than echocardiography. In the second, earlier treated subject, BNP-values were only moderately elevated (86 ng/1) except two determinations timely linked to port implantation. In both patients, BNP levels correlated well with the severity of heart failure when using the NYHA classification modified for infants. These observations illustrate that BNP may be a valuable parameter for surveillance of cardiac function in Pompe's disease.