In this prospective observational study, the incidence, risk factors and the time to event of urinary retention in children receiving intravenous opioids were evaluated. Urinary retention was confirmed by ultrasound following the inability to void for 8 h or earlier in patients experiencing discomfort. In total, 207 opioid episodes were evaluated, of which 199 (96.1%) concerned morphine, in 187 children admitted to the pediatric ward or pediatric intensive care unit. The median age was 7.6 years (IQR 0.9-13.8), and 123 (59.4%) were male. The incidence of urinary retention was 31/207 (15.0%) opioid episodes, in which 14/32 (43.8%) patients received continuous sedation for mechanical ventilation and 17/175 (9.7%) received no sedation. Multivariable logistic regression analysis showed a significant association with continuous sedation (OR 6.8, 95% CI 2.7-17.4, p 0.001) and highest daily fluid intake (OR 0.8 per 10% deviation of normal intake, 95% CI 0.7-0.9, p 0.01). Opioid dosage, age and gender were not significantly associated. Most events (28/31, 90.3%) occurred within 24 h.Conclusion: The incidence of urinary retention in children receiving intravenous opioids is low, indicating that placement of urinary catheters is not routinely necessary in these patients. However, micturition and bladder volumes must be monitored, especially in sedated children and during the first 24 h of opioid administration. What is Known: • Great variation exists in the routine placement of urinary catheters in children receiving IV opioids. What is New: • Confirmed by ultrasound, the incidence of urinary retention in children receiving intravenous opioids in this study was 15%, indicating that placement of urinary catheters is not routinely necessary in these patients. • Children receiving continuous sedation for invasive mechanical ventilation showed a sevenfold greater risk of developing urinary retention than non-sedated patients.