Background ICIN is a major cause of death in cancer patients (pts). We aimed at identifying incidence of ICIN and predictors of poor prognosis in oncology. Methods We designed a multicenter retrospective cohort study on the Greater Paris University Hospitals' (APHP) clinical data warehouse. We included all adult pts with solid cancer hospitalized between January 2016 and December 2021 at APHP with an ICD-10 cancer code, with chemotherapy (chemo) within 30 days prior to the 1st ICIN (defined by the D70 or D611 ICD-10 codes AND a neutrophil count < 500/mm3). The primary outcome was the composite of Intensive Care Unit (ICU) hospitalization or death within 30 days. We collected cancer and chemo characteristics, use of G-CSF, Charlson score items and standard biological values. Results Among 141,586 cancer pts, 40,660 received chemo among whom 661 (1.6%) had ICIN. Their median age was 63 years (Interquartile Range (IQR) 54-70) and 330 (50%) were female. The median Charlson score was 10 (IQR 8-11). Documented bacterial or fungal infections were observed in 113 (17%) and 19 (3%) pts, respectively. Main primary cancers were lung (n= 204, 31%) and breast (n=87, 13%). Advanced cancers were present in 551 (83%) pts. The last chemo included platinum in 375 (57%) pts, 331 (50%) were in their 1st line of chemo and 284 (43%) in their 1st cycle of the current line, 149 (23%) had primary prophylactic G-CSF, 27 (4%) had a concomitant SarsCov2 infection. Primary outcome occurred in 120/661 (18%) pts: 57 (9%) were transferred to intensive care and 82 (12%) died within 30 days. After ICIN, 372 (56%) pts received subsequent chemo and 486 (74%) were able to return home. In a multivariate Cox model, higher monocyte count (HR 0.07; 95%CI 0.01-0.44) and albumin nadir (HR 0.93; 95%CI 0.91-0.96) decreased ICU hospitalization risk or death whereas both bacterial (HR 2.12; 95%CI 1.40-3.31) or fungal infection (HR 1.93; 95%CI 1.40-3.31) and concomitant Sars Cov2 infection (HR 2.93; 95%CI 1.41-6.33) increased it. Charlson index (HR: 1.00; 95%CI 0.91-1.10) and primary prophylactic G-CSF (HR 0.83; 95%CI 0.52-1.32) had no prognostic value. Conclusions Despite the use of primary prophylactic G-CSF, ICIN is an early event which leads to major complications.