Background: The Bosniak classification is used to risk stratify the probability of cancer among cystic renal masses using CT or MRI and assigns a class ranging from I-IV. Bosniak III and IV masses have the highest probabilities of cancer and are treated similarly to solid renal masses per 2021 American Urological Association (AUA) Guidelines. The purpose of this study was to determine the growth kinetics and progression rates of unresected Bosniak III and IV masses defined by Bosniak Classification, version 2019 (v2019). Methods: In this retrospective and IRB-approved study, CT and MRI images of 107 cystic renal masses from 100 patients (34 women, 66 men; range 28-92 years) with unresected Bosniak III and IV masses from 2005-2021 were reviewed. Cystic renal masses on baseline imaging were assigned a category based on v2019 by two abdominal radiologists, with discrepancies resolved by a third, such that each mass had a single Bosniak class assignment. Mass dimensions over serial imaging were measured using orthogonal planes by a radiologist with 3D laboratory training. Linear growth rate (mm/year) and volumetric growth rate (mL/year) were calculated. “Progression” off of active surveillance was defined as any of the following: linear growth rate > 5 mm a year, volume doubling rate ≤ 1 year, or increased stage based on TNM. Results: 107 cystic renal masses were categorized as 51 Bosniak III and 56 Bosniak IV masses on baseline exam. Median follow-up time was 406 days. Linear growth rate for Bosniak III masses was 0.4 ± 4.3 mm/year while linear growth rate for Bosniak IV masses was 2.3 ± 4.9 mm/year (p = 0.036; Student’s t-test). 12/51 (24%) Bosniak III masses progressed based on above definitions while 26/56 (46%) Bosniak IV masses progressed (p = 0.016; Fisher’s exact test). 13 masses upgraded from a Bosniak III to IV during surveillance, but category upgrades were not associated with progression (p = 0.131; Fisher’s exact test). 2 patients developed lung metastases; both masses were Bosniak IV and were biopsy proven papillary and clear cell renal cell carcinomas. Conclusions: Although currently treated similarly by AUA Guidelines, unresected Bosniak IV masses grow faster than Bosniak III masses and are more likely to progress off of active surveillance.