We retrospectively reviewed the medical records of 32 chemonaïve patients with either breast, lung or prostate cancer, who were treated with docetaxel (DOC) monotherapy, and evaluated whether the proportion of peripheral blood monocytes was capable of predicting the occurrence of neutropenia following chemotherapy. In the granulocyte-colony stimulating factor (G-CSF) non‑administration group, the monocyte percentage was inversely correlated with the decrease in neutrophils (P=0.01; corrected correlation coefficient, -0.71). The neutrophil count decreased by ≥30% in 7 of 8 patients with <5% monocytes, whereas it decreased by >30% in 1 of 6 patients with ≥5% monocytes (P=0.01). Three of 8 patients with <5% monocytes experienced grade 4 neutropenia, while in the group with ≥5% monocytes, 1 of 6 patients experienced grade 4 neutropenia. The frequency of grade 3 or 4 neutropenia was lower in patients with ≥5% monocytes than in patients with <5% monocytes, but the difference was not significant (P=0.41). Following G-CSF administration, grade 3 or 4 neutropenia had the tendency of lasting longer in patients with <5% monocytes than in those with ≥5% monocytes; however, the monocyte percentage was not correlated with the grade of neutropenia (P=0.34). The monocyte percentage following chemotherapy was inversely correlated with the decrease in neutrophils. The percentage of monocytes that are available in clinical practice may be predictive of neutropenia following chemotherapy. Our findings suggest that patients with <5% monocytes following DOC monotherapy are at risk of severe neutropenia and should be carefully monitored.

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