PURPOSE Health care consolidation has significantly affected cancer care delivery, with oncology practices undergoing substantial consolidation over the past two decades. This study investigates practice consolidation trends among medical oncologists (MOs), factors associated with consolidation, and changes in MO geographic distribution. METHODS Medicare data from 2015 to 2022 were used to assess MO practice consolidation in hospital referral regions (HRRs), linked with regional health care market data and physician demographics. The Herfindahl-Hirschman Index (HHI) was used to measure consolidation, and the Gini coefficient was used to measure MO distribution across counties. Multivariable linear regression explored factors associated with MO practice consolidation. RESULTS Between 2015 and 2022, the number of MOs increased by 14.5% (11,727-13,433), whereas the number of MO practices decreased by 18.0% (2,774-2,276). The mean number of MOs per practice increased by 40% (4.26-5.95; P < .001). The percentage of MOs in small practices decreased, whereas larger practices saw an increase. MO consolidation, as indicated by the HHI, increased by 9% (median HHI, 0.3204-0.3480). HRRs with higher baseline hospital consolidation and more hospital beds per capita were more likely to have MO practice consolidation. Despite MO practice consolidation, the county-level distribution of MOs did not change substantially. CONCLUSION On the basis of Federal Trade Commission classifications, MO practices were highly concentrated in 2015 and consolidated even further by 2022. While distribution of MOs at the county level remained stable, further research is needed to assess the effects of rapid consolidation on cancer care cost, quality, and access. These data have important implications for policymakers and payers as they design programs that ensure high-quality, affordable cancer care.