Ten consecutive patients with myelofibrosis were examined for the following signs of PNH: Ham’s test, the sucrose hemolysis test, low red cell acetylcholinesterase (ACHE) activity and intravascular hemolysis. Two of the patients displayed all these signs and also had clinical symptoms of PNH. Two patients had positive sucrose hemolysis tests, low red cell ACHE and intravascular hemolysis. One patient had a positive sucrose hemolysis test and low red cell ACHE. The remaining patients had no signs of PNH. In two of the patients, the PNH signs disappeared after splenectomy. The concurrence of myelofibrosis and PNH supports the hypothesis that PNH is a disease of the common hemopoietic stem cell.