The aim of the study was to screen formalin-fixed, paraffin-embedded tissues of polycythemia vera patients for the presence of JAK2(V617F) and JAK2 exon 12 mutations. Of 64 cases, 60 were positive for JAK2(V617F) mutation using allele-specific polymerase chain reaction (PCR). Using modified allele-specific PCR, samples of 4 JAK2(V617F)-negative patients were analyzed for the presence of JAK2 exon 12 mutations. In one case, we found a PCR product matching with allele-specific primer, which was designed to detect the N542-E543del mutation. Surprisingly, in the result sequence we have detected another recently described mutation, R541-E543delinsK. In the other 3 JAK2(V617F)-negative patients, allele-specific PCR for the detection of JAK2 exon 12 mutations did not yield any product. However, in 1 case, the sequencing of JAK2 exon 12 PCR product revealed a novel mutation, H538-K539delinsF. We confirmed that paraffin-embedded tissues represent a valuable source of DNA, which can be used in diagnostics of both JAK2 exon 12 and exon 14 mutations and we described 1 novel JAK2 exon 12 mutation.