Langerhans cell histiocytosis (LCH) is a disorder characterized by an abnormal accumulation of CD207 and CD1a cells in almost any tissue. Currently, there is a lack of prognostic markers to follow up patients and track disease reactivation or treatment response. Putative myeloid precursors CD207 and CD1a cells were previously identified circulating in the blood. Therefore, we aim to develop a sensitive tracing method to monitor circulating CD207 and CD1a cells in a drop of blood sample of patients with LCH. A total of 202 blood samples from patients with LCH and 23 controls were tested using flow cytometry. A standardized cellular score was defined by quantifying CD207 and CD1a expression in monocytes and dendritic cells, based on CD11b, CD14, CD11c, and CD1c subpopulations, resulting in a unique value for each sample. The scoring system was validated by a receiver operating characteristic curve showing a reliable discriminatory capacity (area under the curve of 0.849) with a threshold value of 14, defining the presence of circulating CD207 and CD1a cells. Interestingly, a fraction of patients with no evident clinical manifestation at the time of sampling also showed presence of these cells (29.6%). We also found a differential expression of CD207 and CD1a depending on the organ involvement, and a positive correlation between the cellular score and plasma inflammatory markers such as soluble CD40L, soluble IL-2Ra, and CXCL12. In conclusion, the analysis of circulating CD207 and CD1a cells in a small blood sample will allow setting a cellular score with minimal invasiveness, helping with prognostic accuracy, detecting early reactivation, and follow-up.