Most children in foster care have poor health outcomes and high health care utilization. Health complexity influences health care utilization as well foster care placement. Health care utilization studies among children in foster care have not accounted for health complexity status and foster placement. A 7-year retrospective study linked Colorado child welfare and Medicaid administrative data for 30,164 Medicaid-enrolled children, up to 23 years old, who differed by initial foster care entry, to examine primary care and behavioral health (BH) utilization patterns from 2014 to 2021. Children entering care were matched with replacement to non-foster peers by age, sex, Medicaid enrollment patterns, managed care status, family income, and health complexity. We calculated weighted monthly average percentages of children with primary care and BH utilization by foster care entry, health complexity, sex, and age over 25 months relative to the month of foster care entry for the foster cohort or the reference month for non-foster peers. Children in the foster cohort had lower primary care but higher BH utilization relative to non-foster peers prior to the reference month. Primary care and BH use increased among children in foster care during and 12 months after the reference month, unlike matched comparisons. Primary care and BH utilization increased by health complexity but differed by foster care status and time. Foster care entry and health complexity produced distinct patterns of primary care and BH utilization. Given higher utilization among children in foster care, future investigation should explore health care quality and delivery factors.