To evaluate ambulatory-care sensitive (ACS) hospitalizations for children with noncomplex chronic diseases (NC-CD) and children with medical complexity (CMC), and identify associations with ambulatory care characteristics. Although ACS hospitalizations are potentially preventable in general populations, the specific ambulatory care predictors and influence of medical complexity on them is poorly understood. Retrospective cohort study of NC-CD and CMC hospitalizations at a children's hospital during 2007-2014, excluding labor/delivery and children over 21 years. Pediatric medical complexity algorithm identified NC-CD or CMC. ACS hospitalizations were identified using Agency for Healthcare Research and Quality indicator definitions. Demographic and ambulatory care characteristics were compared between ACS and non-ACS hospitalizations with logistic regression clustered by patient. Measures of ambulatory care during 2 years before admission were explored with 20% random sample of general pediatrics discharges. Among 4035 children with NC-CD, 14.6% of 4926 hospitalizations were ACS hospitalizations. Among 5084 CMC, 5.3% of 14 390 discharges were ACS hospitalizations. Among NC-CD discharges, ACS hospitalizations were more likely with no prior-year outpatient visits (OR 1.4, 95% CI 1.1-1.7) and less likely with timely well checks (OR 0.8, 95% CI 0.6-0.9) and phone encounters in the month before admission (OR 0.5, 95% CI 0.2-1.0). Among CMC discharges, the only association observed was with provider continuity (OR 0.3, 95% CI 0.1- 1.0). Provider continuity may be associated with fewer CMC ACS hospitalizations, however, measures of ambulatory care were more consistently associated with ACS hospitalizations for NC-CD. CMC may need more precise ACS hospitalization definitions.