ABSTRACT Objectives: to estimate the incidence and the risk factors for severe acute postoperative pain self-reported on the first day after hospital discharge. Methods: cohort study with 279 children from both sexes (5-12 years old), indicated for ambulatory surgery in two Brazilian hospitals. Children were assessed at the pre-surgery, immediate postoperative and first postoperative day. Faces Pain Scale-Revised and Yale Preoperative Anxiety Scale Modified were used. Severe postoperative pain was defined as score ≥6. Cox regression analyses were used. Results: the incidence of severe postoperative pain was 15.8% (95%CI:10.7%-20.4%) on the first postoperative day. Preoperative anxiety (HR=2.23; p=0.049), severe preoperative pain (HR=2.78; p=0.031) and having undergone two surgical procedures (HR=2.91; p=0.002) were associated with severe postoperative pain. Conclusions the incidence of severe postoperative pain self-reported after hospital discharge was high. Anxiety and severe preoperative pain, in addition to performing two surgical procedures at the same time were confirmed as risk factors.