Summary. Background: Indwelling arterial catheters (IACs) are used for monitoring and blood sampling purposes in intensive care units. Very limited information is available on the incidence and risk factors of IAC-related thrombosis in children. Objective: To investigate the incidence and predictors of IAC-related thrombosis in a tertiary care pediatric hospital. Methods: For a period of 12 months, detailed information was prospectively recorded for all consecutive children requiring IACs. Results: Six hundred and fifteen IACs were placed in a total of 473 children at a median age of 0.56 years for a total of 47 440.84 catheter hours. Of the 615 IACs, 418 (68%) were placed in the radial artery, 137 (22%) in the femoral artery, 26 (4%) in the umbilical artery, 11 (2%) in the brachial artery, and 23 (3.7%) in another artery. Thrombosis occurred in 20 cases, reflecting an overall incidence of 3.25%. Eighteen of the 20 IAC-related thrombi were located in the femoral arteries, reflecting a relative incidence of 13% (18/137). Newborn age, lower body weight, low cardiac output and increased hematocrit were significantly related with an increased risk of femoral artery thrombosis. In logistic regression analysis, younger age (P < 0.001, odds ratio 6.51) was independently associated with an increased thrombotic risk. Conclusions: This study demonstrates that arterial thrombosis occurs with an increased incidence in children requiring IACs in the femoral location. Younger age is independently associated with an increased risk of thrombosis. The radial location is safe, and should be preferred to the femoral location.