Background: Bosniak classification version 2019 (v2019) was a major revision to version 2005 (v2005) that defined cystic renal mass subclasses based on wall or septa features. Objective: To determine the proportion of malignancy within cystic renal masses stratified by Bosniak classification v2019 class and feature-based subclass. Evidence Acquisition: MEDLINE and EMBASE databases were searched on July 24, 2023 for studies published in 2019 or later that reported cystic renal masses that underwent renal-mass CT or MRI, were assessed using Bosniak v2019, and had a reference standard (histopathology indicating benignity or malignancy or ≥5-year imaging follow-up indicating benignity). Study authors were contacted to provide subclass-stratified data. Pooled proportions of malignancy stratified by v2019 class and subclass were determined using meta-analysis. Evidence Synthesis: The analysis included 12 studies reporting 966 patients with 975 cystic masses. No class I mass was malignant. Pooled proportions of malignancy by class were: II, 9% (95% CI: 5-17%); IIF, 26% (95% CI: 13-46%); III, 80% (95% CI: 71-87%); IV, 88% (95% CI: 83-91%). Pooled proportions of malignancy by subclass were: IIF with many smooth, thin septa, 10% (95% CI: 2-33%); IIF with minimal wall or septal thickening, 47% (95% CI: 18-77%); IIF with heterogeneous T1-weighted hyperintensity, 26% (95% CI: 8-57%); III with thick smooth wall or septa, 78% (95% CI: 60-90%); III with obtuse protrusion(s) ≤3 mm, 84% (95% CI: 77-90%); IV with acute protrusion(s) of any size, 88% (95% CI: 80-93%); IV with obtuse protrusion(s) ≥4 mm, 86% (95% CI: 77-91%). Proportion of malignancy was 41% for IIF masses with histopathology reference versus 2% for IIF masses with imaging follow-up reference. In four studies performing intraindividual comparisons of v2005 versus v2019, proportion of malignancy was: IIF, 24% versus 42% (p=.13); III, 74% versus 77% (p=.72); IV, 79% versus 84% (p=.22) Conclusion: Bosniak IIF masses had higher malignancy rates when histopathology rather than imaging follow-up was the reference standard, indicating verification bias. All Bosniak III and IV subclasses had high malignancy rates. Clinical Impact: The results improve understanding of imaging-based cystic renal mass classification and may inform development of future renal mass classification systems.