Background: Chemotherapy-induced neutropenia (CIN) is chemotherapy's most frequent myelosuppression effect. Previous studies have examined static prognostic factors to assess dynamic neutrophil changes in each cycle. This study focuses on post-chemotherapy monocytopenia as a more dynamic predictor of CIN. This research aimed to know whether post-chemotherapy monocytopenia can be used as a predictor of CIN in breast cancer patients undergoing the chemotherapy cycle. Methods: This research used a prospective and a retrospective longitudinal cohort of breast cancer patients undergo a CAF regimen at Sanglah Hospital Denpasar. The research was conducted on 30 samples for three cycles (90 total cycles). Monocyte data is taken in the first week (days 4 to 9), and neutrophil data is taken in the second week (days 10 to 16). Results: From statistical analysis with interval data scale, we obtained that p 0.001 with Pearson correlation 0.613 for cycle 1, p 0.001 with Pearson correlation 0.611 for cycle 2, and p 0.003 with Pearson correlation 0.522 for cycle 3. Conclusions: In conclusion, post-chemotherapy monocytopenia can be used to predict the occurrence of chemotherapy-induced neutropenia (CIN) with the strongest positive correlation in the first cycle.