Fractures are the second most common abusive injury occurring in young children, particularly those under 2 years of age. The humerus is often affected. To better identify factors discriminating between abusive and non-abusive humerus fractures, this retrospective study examined the characteristics and mechanisms of injuries causing humerus fractures in children less than 18 months of age. Electronic medical records were reviewed for eligible patients evaluated between September 1, 2007 and January 1, 2012 at two children's hospitals in Chicago, IL. The main outcome measures were the type of fracture and the etiology of the fracture (abuse vs not abuse). The 97 eligible patients had 100 humerus fractures. The most common fracture location was the distal humerus (65%) and the most common fracture type was supracondylar (48%). Child Protection Teams evaluated 44 patients (45%) and determined that 24 of those had 25 fractures caused by abuse (25% of the total study population).Among children with fractures determined to have been caused by abuse, the most common location was the distal humerus (50%) and the most common types were transverse and oblique (25% each); however, transverse and oblique fractures were also seen in patients whose injuries were determined to have been non-abusive. A younger age, non-ambulatory developmental stage, and the presence of additional injuries were significantly associated with abusive fractures. Caregivers did not provide a mechanism of injury for half of children with abusive fractures, whereas caregivers provided some explanation for all children with non-abusive fractures.