Polycythemia vera - the most frequent form of a primary erythrocytosis in adult patients - represents an extremely rare disease in pediatric and juvenile patients as do congenital primary and secondary erythrocytoses. Frequently, in patients with these diseases clinical problems do not occur before adulthood. Systematic data on clinical and laboratory evaluations as well as on treatment regimens are sparse. In addition, in the majority of cases with congenital erythrocytosis, the etiology is unknown. For those reasons, a protocol (PV-ERY-KA 03) for the systematic collection of clinical, hematological, biochemical, biological as well as treatment data of children and adolescents with polycythemia vera or congenital erythrocytosis including the hemoglobinopathies with high affinity hemoglobin, familial 2,3-BPG deficiencies, and those of unknown origin, has been developed. These data are combined with molecular analyses which focus initially on EPO-receptor and vHL-gene examination, but will later be extended into presently unexplored pathophysiologic regulatory circuits. In addition, pathophysiologic changes due to the erythrocytosis will be studied. The co-ordinated medical care for patients with those rare diseases within a collaborative trial accompanied by scientific projects is aimed at the improvement of the treatment of these patients as well as to a better understanding of the underlying biological processes.