Juvenile haemochromatosis is a rare inborn error of iron metabolism with clinical manifestations before 30 years of age. Unlike adult haemochromatosis which principally affects men, juvenile haemochromatosis affects the sexes equally; it causes early endocrine failure, dilated cardiomyopathy and joint disease. We report four patients (two of each sex) from three pedigrees affected by juvenile haemochromatosis with a mean onset at 22 years (range 14-30). All had endocrine deficiency with postpubertal gonadal failure secondary to pituitary disease; two suffered near-fatal cardiomyopathy with heart failure. Mean time to diagnosis from the first clinical signs of disease was 9.8 years (range 0.5-20) but general health and parameters of iron storage responded favourably to iron-depletion therapy. A 24-year-old man listed for heart transplantation because of cardiomyopathy [left ventricular (LV) ejection fraction 16%] responded to intravenous iron chelation with desferrioxamine combined with phlebotomy (ejection fraction 31%). A 27-year-old woman with subacute biventricular heart failure refractory to medication required orthotopic cardiac transplantation before the diagnosis was established (LV ejection fraction 25%). Genetic studies showed that these two patients with cardiomyopathy from unrelated families were heterozygous for the HFE 845G-->A (C282Y) mutation and wild-type at the H63D locus: complete sequencing of the intron-exon boundaries and entire coding sequence of the HFE gene failed to identify additional lesions. Two siblings in a pedigree without cardiomyopathy were wild-type at the HFE C282Y locus; although the brother harboured a single copy of the 187C-->G (H63D) allele, segregation analysis showed that in neither sibling was the iron-storage disease linked to MHC Class I markers on chromosome 6p. Juvenile haemochromatosis is thus a genetically heterogenous disorder distinct from the common adult variant.