[Polycythemia in kidney cysts. A report on an unusual case].

A 43-year-old woman with a solitary renal cyst developed polycythaemia. 21 well documented cases of this type have been previously published. In the majority of them a causal connection could be assumed between the renal cyst and polycythaemia: in 13 of the 15 cases (as in the reported one), the polycythaemia disappeared after surgical removal of the cyst. Measurement of serum erythropoietin can help in the differential diagnosis, but exclusion of polycythaemia vera may be difficult in the individual case.