The purpose of this investigation was to study the influence of the flare at the lateral side of the heel of running shoes on: initial and total pronation; impact forces in heel-toe running; and to explain the results with a mechanical model. The experimental part of the study was performed by using 14 male runners. Their running movement (4 m/s) was quantified by using a force platform and high-speed film (100 frames X s-1). Three shoes were used, identical except in their lateral heel flare, one shoe with a conventional flare of 16 degrees, a second shoe with no flare, and a third shoe with a rounded heel (negative flare). The experimental results indicate that (for the used set of shoes); increasing heel flare increases the amount of initial pronation; changes in heel flare do not affect the magnitude of the total pronation; and changes in heel flare do not alter the magnitude of the impact force peaks. Since shoes with rounded lateral heels do reduce initial pronation, it is speculated that this construction could be used to prevent anterior medial compartment syndrome at the tibia of runners. It was concluded that more research is needed to specify whether the reported result is representative for various shoe types or is shoe specific.