Abstract: Pontomedullary lacerations (PMLs) have often been reported in car occupants and pedestrians, are less frequently described in motorcyclists, and are very rarely described in bicyclists. The aim of this study was to determine the frequency of brainstem PMLs among fatally injured motorcyclists and bicyclists as well as the frequency of concomitant cranial, facial, and cervical spine injuries in such cases. A possible underlying mechanism of PML in fatally injured motorcyclists and bicyclists might thus be established. Of 443 cases of fatally injured motorcyclists and bicyclists, a sample of 381 cases of fatally injured motorcyclists and bicyclists with head injury of Abbreviated Injury Scale score of 3 or greater was formed and further analyzed. This group was composed of 345 men and 36 women. The average age was 48.8 ± 20.8 years (range, 15–99 years). In the analyzed sample group, there were 158 motorcyclists and 223 bicyclists. Partial PMLs were present in 44 cases (12%) within the sample of 381 head injuries, which breaks down to 40 men and 4 women. In our study, the impact area on the head and the specific skull base fracture type were good predictors of either PML occurrence or absence (B = −2.036, Wald = 161.312, P < 0.01, for the whole model). Impact to the chin, with or without a skull base fracture, most often led to this fatal injury due to impact force transmission, either through jawbone or vertebral column. Also, lateral head impact, the most frequent in bicyclists, with subsequent hinge fracture, PML, and frontoposterior hyperextension of the head that is associated with upper spine fracture, could be possible mechanisms of brainstem injury in fatally injured motorcyclists or bicyclists. Our study showed that the jawbone, as well as other facial bones, could act as shock absorbers, and their fracture could diminish energy transfer toward the skull and protect the brain and brainstem from injury.