Background: Adverse events (AEs) during neoadjuvant chemotherapy (NAC) for muscle invasive bladder cancer (MIBC) are known but insufficiently reported. Clinical implications include affected cardiac, pulmonary, urinary, vascular and haematological organ systems. The main purpose was to evaluate the incidence and severity of AEs for ascertaining possible clinical significance. Further investigating possible effects of AEs on downstaging outcomes-downstaging is considered a surrogate marker for overall survival (OS).Methods: A retrospective evaluation of AEs during ongoing NAC for MIBC patients analysing individual patient data in a clinical database. We identified 687 cystectomies between 2009-2020 at four Swedish urological centres. Inclusion criteria were cT2-4aN0M0 in 261 NAC patients undergoing radical cystectomy (RC). Medical files were reviewed and AEs were assessed and graded, including detailed measurements by the Common Terminology Criteria for Adverse Events (CTCAE) v.5. Data were retrospectively analysed in SPSS statistics 27.0 with Spearman rank-order correlation coefficient and Mann-Whitney U-test (MWU).Results: A total of 251/261 patients [95% confidence interval (CI), 93-98%] experienced AEs during NAC pre-RC (mean two AEs/patient). In total, 208 (80%) patients received methotrexate, vinblastine, adriamycin (doxorubicin) and cisplatin (MVAC). In the total cohort, 200 (76.6%) received all pre-planned NAC-cycles. Most common AEs were anaemia (88.9%), thrombocytopenia (44.8%) and acute kidney injury (40.6%). Patients with prematurely terminated cycles had higher AE-grades (P=0.042 MWU). A correlation between higher AE-grades and decrease in downstaging existed, in the entire cohort (-0.133; P=0.033) and in patients undergoing all pre-planned NAC-cycles (-0.148; P=0.038). Anaemia and acute kidney injury were individually associated with decreased downstaging (-0.360, P=0.025 and -0.183, P=0.010, respectively).Conclusion: NAC in MIBC poses a significant risk for AEs before RC with clinical implications. For instance, patients terminating chemotherapy prematurely, have higher AE-grades and decreased downstaging. Further, acute kidney injury and anaemia are individually associated with decreased downstaging. We propose that early detection and prevention of AEs may increase downstaging of the primary tumour.