Chronic myelomonocytic leukemia is characterized by persistent absolute monocytosis (≥1 × 10(9)/l) in the peripheral blood and dysplasia in ≥1 lineages. In the absence of dysplasia, an acquired clonal genetic abnormality is required or causes for reactive monocytosis have to be excluded. Oligomonocytic chronic myelomonocytic leukemia showing increased monocytes but no absolute monocytosis in the peripheral blood occurs occasionally. These cases are likely classified as myelodysplastic syndrome or myelodysplastic/myeloproliferative neoplasm, unclassifiable. A subset eventually develop overt chronic myelomonocytic leukemia. Better characterization of oligomonocytic chronic myelomonocytic leukemia is essential since the distinction between chronic myelomonocytic leukemia and myelodysplastic syndrome is clinically relevant. We identified 44 cases of oligomonocytic chronic myelomonocytic leukemia (≥10% peripheral blood monocytes with absolute monocyte count of 0.5-1 × 10(9)/l) and 28 consecutive chronic myelomonocytic leukemia controls. Clinicopathologic features were compared and mutation analysis was performed. Oligomonocytic chronic myelomonocytic leukemia patients were significantly younger (median age of 65 vs 72). They had lower WBC and absolute neutrophil count, while the monocyte percentage, hemoglobin and platelet counts were similar in the two groups. The myeloid to erythroid ratio was predominantly decreased or normal, compared with the characteristic increase in chronic myelomonocytic leukemia (P=0.006). 38% of patients progressed to overt chronic myelomonocytic leukemia (median: 12 months). The overall percentage of mutations was significantly lower in oligomonocytic chronic myelomonocytic leukemia. However, the most frequent mutations in both groups were the 'signature' chronic myelomonocytic leukemia mutations in ASXL1, TET2 and SRSF2. Mutations in CBL were found exclusively in overt chronic myelomonocytic leukemia. In conclusion, we demonstrate clinical and genetic similarities between overt chronic myelomonocytic leukemia and oligomonocytic chronic myelomonocytic leukemia. The findings suggest that at least a subset of oligomonocytic chronic myelomonocytic leukemia represents early phase 'dysplastic type' chronic myelomonocytic leukemia.