1555 Background: Thrombocytopenia is a common toxicity of chemotherapy, yet there are limited data on its occurrence in routine clinical practice. Methods: Using structured patient-level data from the Flatiron Health EHR-derived database, we assessed risk (3-month cumulative incidence) of chemotherapy-induced thrombocytopenia (CIT) in adult patients (2012-2017) based on platelet counts, overall and by each grade of CIT, cancer type, and chemotherapy regimen (Table); and the co-occurrence of other hematologic abnormalities. Results: Of 15,521 solid tumor patients who initiated chemotherapy, 13% had evidence of CIT within 3 months (platelet count <100x109/L), 4% had grade 3 (25 to <50x109/L) and 2% had grade 4 (<25x109/L) CIT. Of the solid tumors examined, incidence was highest in melanoma patients. In hematologic malignancies (N = 2,537), 3-month risk was even higher with nearly 30%, 16%, and 12% having any grade, grade 3 and 4 CIT, respectively; and the greatest risk being in multiple myeloma patients. Anthracycline-based regimens were associated with the highest risk of CIT (7% grade 3; 4% grade 4), followed by gemcitabine- and platinum-based regimens. Anemia often accompanied first evidence of CIT (49%); isolated thrombocytopenia occurred in 15%. Conclusions: This study provides a current snapshot of CIT risk in a large sample of adult patients undergoing chemotherapy in routine clinical practice, highlighting patients at highest risk for CIT and underscoring the complexity of managing cancer treatment. [Table: see text]