After accounting for misdiagnosis and treatment effect, allele-specific (AS)-PCR detects the JAK2V617F mutation in >95% of polycythemia vera (PV) patients. Using database inquiry, we identified 6 of a total 220 cases with PV that were JAK2V617F-negative (prevalence=3%). Of these, five cases ( approximately 80%) were found to harbor one of the two JAK2 exon 12 mutations (F537-K539delinsL or N542-E543del) in bone marrow (BM) and/or peripheral blood cells. Similar screening of six additional cases - three each with idiopathic erythrocytosis (IE) or otherwise unexplained erythrocytosis (UE) - did not reveal either JAK2V617F or JAK2 exon 12 mutations. We found JAK2 exon 12 mutations in PV cases to be readily detected by both DNA sequencing and AS-PCR, regardless of whether BM or peripheral blood cells were used as the source for DNA. Although erythroid hyperplasia was the predominant histologic feature on BM examination, megakaryocyte abnormalities and reticulin fibrosis were noted in most PV patients harboring exon 12 mutations. However, similar BM morphologic changes can also be seen in some JAK2V617F-positive PV cases; therefore, distinct genotype-phenotype association cannot be established.