As of August 2022, clusters of acute severe hepatitis of unknown etiology in children have been reported from 35 countries, including the United States(1,2). Previous studies have found human adenoviruses (HAdVs) in the blood from cases in Europe and the United States(3-7), although it is unclear whether this virus is causative. Here we used PCR testing, viral enrichment based sequencing, and agnostic metagenomic sequencing to analyze samples from 16 HAdV-positive cases from October 1, 2021 to May 22, 2022, in parallel with 113 controls. In blood from 14 cases, adeno-associated virus 2 (AAV2) sequences were detected in 93% (13 of 14), compared to 4 (3.5%) of 113 controls (P<0.001) and to 0 of 30 patients with hepatitis of defined etiology (P<0.001). In controls, HAdV-41 was detected in blood from 9 (39.1%) of the 23 patients with acute gastroenteritis (without hepatitis), including 8 of 9 patients with positive stool HAdV testing, but co-infection with AAV2 was observed in only 3 (13.0%) of these 23 patients versus 93% of cases (P<0.001). Co-infections by Epstein-Barr virus (EBV), human herpesvirus 6 (HHV-6), and/or enterovirus A71 (EV-A71) were also detected in 12 (85.7%) of 14 cases, with higher herpesvirus detection in cases versus controls (P<0.001). Our findings suggest that the severity of the disease is related to co-infections involving AAV2 and one or more helper viruses.