We examined the efficacy of long-term subcutaneous deferoxamine therapy in the prevention of iron-related cardiac disease in patients with thalassemia major who began treatment after the age of 10 years. Of 36 such patients without preexisting cardiac disease, 19 did not comply with the program of chelation therapy. Over the course of treatment (1977 to 1983) serum ferritin and aspartate aminotransferase levels fell in the compliant group, from mean values (+/- S.D.) of 4765 +/- 2610 to 2950 +/- 1850 ng per milliliter and 58.1 +/- 22 IU to 30 +/- 20 IU per liter, respectively (P less than 0.05), but rose in the noncompliant group, from 5000 +/- 2316 to 6040 +/- 2550 ng per milliliter and 56.6 +/- 20 to 90 +/- 35 IU per liter, respectively. Only one patient in the compliant group acquired cardiac disease and died of fulminant congestive heart failure. In contrast, 12 noncompliant patients acquired cardiac disease, and 7 died. In addition, the mean age of the compliant population (18.9 +/- 4.5 years) now approaches the mean age of acquisition of cardiac disease in the noncompliant group (19 +/- 4.3). These data demonstrate that compliance with treatment with deferoxamine may protect patients from cardiac disease induced by iron overload.