Background: It is generally believed that certain patient characteristics (e.g., Body Mass Index and age) predict difficulty of intravenous cannulation in children, but there is not much literature evaluating these risk factors. In this study, we investigated predictive factors for success rate at first attempt and time needed for intravenous cannulation. Methods/Materials: In a prospective cohort study, we observed characteristics of intravenous cannulations in pediatric patients at the operating room (n = 1083) and the outpatient care unit (n = 178) of a tertiary referral pediatric hospital. Time to successful intravenous cannulation, success at first attempt, and potential predictors for difficult cannulation (age, gender, skin color, BMI or weight-to-age z-score, the child being awake or anesthetized, operator profession and surgical specialty) were recorded. Regression models were constructed to find significant predictors. Results: Success at first attempt was 73% and 81%, respectively. In the operating room age, operator and surgical specialty were predictive for a successful first attempt and time to successful cannulation. No significant predictive factors were found for the outpatient care unit. BMI or weight-to-age was not related to difficult intravenous cannulation. Conclusions: This study shows that in one-fifth to one-third of the patients, intravenous cannulation required more than one attempt. It is difficult to predict with accuracy the difficulty of intravenous cannulation solely with easily obtainable patient characteristics. © 2011 Blackwell Publishing Ltd.