In the absence of a cytogenetic abnormality or overt dysplasia, chronic myelomonocytic leukemia (CMML) may be difficult to be distinguished from reactive monocytosis. We have previously described a typical growth pattern in CMML patients, i.e., 'pseudonormal' colonies resembling granulocytic colonies but consisting entirely of monocytic cells when stained. To study the utility of the colony forming unit cell assay (CFU-C) as a diagnostic tool in patients with monocytosis, we analyzed a cohort of 48 consecutive patients referred to our institution with peripheral blood monocytosis. Thirty-six patients fulfilled the WHO criteria for CMML; 12 were diagnosed with reactive monocytosis. Of the patients with CMML, 28 showed pseudonormal growth with or without leukemic cluster growth, another four showed exclusively leukemic growth. None of the patients with reactive monocytosis showed either leukemic or pseudonormal growth. With a specificity of 100% and a sensitivity of 89%, the CFU-C assay has a unique potential to distinguish CMML from reactive monocytosis.