Aim: To determine incidence, aetiology, and clinical features of subdural haem atoma and effusion (SDH/E) in infancy throughout the British Isles. Methods: Cas es were notified to the British Paediatric Surveillance Unit over 12 months by p aediatricians, neurosurgeons, and paediatric and forensic pathologists. Results: A total of 186 infants (121 boys, 65 girls) aged 0-2 years were identified. An nual incidence of SDH/E for the UK and Republic of Ireland is 12.54/100 000 aged 0-2 (95%CI 10.3 to 14.62) and 24.1/100 000 aged 0-1 (95%CI 20.89 to 28.18). A total of 106 infants suffered non-accidental head injury (NAHI), 7 accidenta l head injury, 26 a perinatal cause, 7 a non-traumatic medical condition, 23 me ningitis, and in 17 the cause was undetermined; 35 infants died. Significant dif ferences were found in injury pattern, body weight, and Townsend score between N AHI and SDH/E from other cause. There were fewer diagnostic investigations in no n-NAHI cases. Delay in diagnosis of greater than a week occurred in 48/181. Con clusion: SDH/E is a significant cause of morbidity and mortality in infancy. NAH I is the predominant cause of SDH/E. SDH/E can present in a non-specific and va ried way and must be considered in any infant who is unwell. Determining the cau se of the SDH/E in some cases continues to present a diagnostic challenge. Aim: To determine incidence, aetiology, and clinical features of subdural haem atoma and effusion (SDH / E) in infancy throughout the British Isles. Methods: Cas es were notified to the British Pediatric Survillance Unit over 12 months by pediatricians, neurosurgeons, An incidence of SDH / E for the UK and Republic of Ireland is 12.54 / 100 000 aged 0- A total of 106 infants recovered non-accidental head injury (NAHI), 7 accidental head injury, 26 a (95% CI 10.3 to 14.62) and 24.1 / 100 000 aged 0-1 (95% CI 20.89 to 28.18) perinatal cause, 7 a non-traumatic medical condition, 23 me ningitis, and in 17 the cause was undetermined; 35 infants died. Significant dif ferences were found in injury pattern, body weight, and Townsend score between N AHI and SDH / E from There were fewer diagnostic investigations in no n-NAHI cases. Delay in diagnosis of grea Con clusion: SDH / E is a significant cause of morbidity and mortality in infancy. NAH I is the predominant cause of SDH / E. SDH / E can present in a non-specific and va ried way and must be be considered in any infant who is unwell. Determining the cau se of the SDH / E in some cases continues to present a diagnostic challenge.