A first step toward understanding whether pediatric medical subspecialists are meeting the needs of the nation's children is describing rates of use and trends over time. To quantify rates of outpatient pediatric medical subspecialty use. This repeated cross-sectional study of annual subspecialist use examined 3 complementary data sources: electronic health records from PEDSnet (8 large academic medical centers [January 1, 2010, to December 31, 2021]); administrative data from the Healthcare Integrated Research Database (HIRD) (14 commercial health plans [January 1, 2011, to December 31, 2021]); and administrative data from the Transformed Medicaid Statistical Information System (T-MSIS) (44 state Medicaid programs [January 1, 2016, to December 31, 2019]). Annual denominators included 493 628 to 858 551 patients younger than 21 years with a general pediatric visit in PEDSnet; 5 million beneficiaries younger than 21 years enrolled for at least 6 months in HIRD; and 35 million Medicaid or Children's Health Insurance Program beneficiaries younger than 19 years enrolled for any amount of time in T-MSIS. Calendar year and type of medical subspecialty. Annual number of children with at least 1 completed visit to any pediatric medical subspecialist in an outpatient setting per population. Use rates excluded visits in emergency department or inpatient settings. Among the study population, the proportion of girls was 51.0% for PEDSnet, 51.1% for HIRD, and 49.3% for T-MSIS; the proportion of boys was 49.0% for PEDSnet, 48.9% for HIRD, and 50.7% for T-MSIS. The proportion of visits among children younger than 5 years was 37.4% for PEDSnet, 20.9% for HIRD, and 26.2% for T-MSIS; most patients were non-Hispanic Black (29.7% for PEDSnet and 26.1% for T-MSIS) or non-Hispanic White (44.9% for PEDSnet and 43.2% for T-MSIS). Annual rates for PEDSnet ranged from 18.0% to 21.3%, which were higher than rates for HIRD (range, 7.9%-10.4%) and T-MSIS (range, 7.6%-8.6%). Subspecialist use increased in the HIRD commercial health plans (annual relative increase of 2.4% [95% CI, 1.6%-3.1%]), but rates were essentially flat in the other data sources (PEDSnet, -0.2% [95% CI, -1.1% to 0.7%]; T-MSIS, -0.7% [95% CI, -6.5% to 5.5%]). The flat PEDSnet growth reflects a balance between annual use increases among those with commercial insurance (1.2% [95% CI, 0.3%-2.1%]) and decreases in use among those with Medicaid (-0.9% [95% CI, -1.6% to -0.2%]). The findings of this cross-sectional study suggest that among children, 8.6% of Medicaid beneficiaries, 10.4% of those with commercial insurance, and 21.3% of those whose primary care is received in academic health systems use pediatric medical subspecialty care each year. There was a small increase in rates of subspecialty use among children with commercial but not Medicaid insurance. These data may help launch innovations in the primary-specialty care interface.