This study investigated the relationship between abomasal hairballs (trichobezoars) and perforating abomasal ulcers in unweaned beef calves <4 mo of age. The calves enrolled in the study represented routine necropsy submissions to veterinary practitioners in western Canada and to the Western College of Veterinary Medicine. Regardless of the cause of death, each calf was examined for evidence of abomasal ulcers and hairballs. Thirty-two practitioners and the Western College of Veterinary Medicine provided 166 cases for the study; 56 died of perforating ulcers (ulcer calves), and 110 died of causes unrelated to abomasum (non-ulcer calves). The calves ranged in age from 1-90 d; 154 (92.8%) were <61 d of age. Overall, ulcer calves were 2.74 (P = 0.003) times more likely to die with an abomasal hairball than were the nonulcer calves. However, stratifying the calves into 2 age groups, young (<31 d) and old (>30 d), yielded conflicting results. While the young ulcer calves were 3.81 (P = 0.003) times more likely to have a hairball than were the young nonulcer calves, there was no statistically significant relationship (OR = 0.76, P = 0.65) between ulcer and hairballs in the older calves. The authors concluded that the relationship between hairballs and ulcers in the young calves was probably spurious, created by a Berkson's bias. It is unlikely that abomasal hairballs have a significant role in the development of fatal perforating ulcers in beef calves.