To elucidate the relevance of genetic alterations, we analysed 17 genes known to be involved in haematological neoplasms in patients with chronic leucocytosis and patients with persistent thrombocytosis. Mutations of the JAK2, SETBP1 and ASXL1 genes were found in 1/13, 1/13, and 2/13 patients with leucocytosis, respectively. Mutations of the JAK2, CALR, SETBP1 and ASXL1 genes were found in 1/5, 1/5, 1/5 and 2/5 patients with thrombocytosis, respectively. One leucocytosis patient with a JAK2 V617F mutation developed polycythaemia vera. Another leucocytosis patient developed Philadelphia chromosome-negative chronic myeloid leukaemia (Ph(-) CML) accompanied by t(9;12)(q34.1;p13.?3) (Mori et al. 2016). Another leucocytosis patient with mutations of the SETBP1 and ASXL1 genes progressed to blast crisis of Ph(-) CML accompanied by i(17)(q10). Chronic leucocytosis patients who had genetic alterations tended to develop haematological neoplasms, while thrombocytosis unexpectedly resolved in two persistent thrombocytosis patients with genetic alterations.