Familial juvenile nephronophthisis (NPH) is a hereditary form of chronic tubulointerstitial nephritis with onset in childhood. About one-third of patients develop anaemia before renal insufficiency. We investigated the pathogenetic mechanisms leading to anaemia by comparing 6 patients with NPH and 12 reference patients with other renal diseases. We studied their iron metabolism and measured transferrin receptor-ferritin ratios. There was no evidence for iron deficiency or haemolysis. The serum erythropoietin concentrations of the patients with NPH (12 +/- 2.3 U/I) were low compared with the 12 reference patients (25 +/- 18.9 U/I). In the 2 patients with NPH who were fully investigated, the pharmacokinetics of recombinant human erythropoietin appeared normal. Thus, anaemia in patients with NPH does not result from iron deficiency or correlate with impaired iron status. The mechanism underlying the anaemia of NPH appears to affect the function or regulation of the cells producing erythropoietin.