The efficacy of a noninvasive hemoglobin monitoring device (Astrin, Sysmex, Kobe, Japan) was evaluated for healthy volunteers and for patients with hematologic disorders. At the same time, the effects of WBC counts on noninvasive monitoring were studied by clinical evaluation and in ex vivo experiments. The hemoglobin levels determined by the device (Ast-Hb) and a conventional analyzer (T-Hb) were compared. The coefficient of correlation between findings with the Ast-Hb and the T-Hb for healthy volunteers was r = 0.626, whereas that for patients with hematologic disorders was r = 0.762. A comparison of the ratios of measurement errors in hemoglobin levels by Ast-Hb and T-Hb indicated that the number of WBCs had no effect on hemoglobin monitoring. Moreover, ex vivo studies using isolated WBCs and an optical model that imitates blood vessels and tissue in human fingers confirmed these results. Therefore, this new hemoglobin monitoring device can be expected to be useful for continuous hemoglobin monitoring.