The primary aim was to relate information about masseter muscle fibres and function to aspects of facial morphology in a group of healthy young men. The secondary aim was to investigate possible sex differences using data previously obtained from a comparable group of age-matched, healthy women. Dental status and facial morphology were recorded in 13 male students aged 20-26 years. Functional examinations included bite-force measurements and electromyographic recordings of masseter activity. A biopsy was removed from the masseter of each participant during surgical extraction of a wisdom tooth, and the tissue examined for myosin ATPase activity. Further, the cross-sectional areas of the different fibre types were measured. In spite of using age-matched healthy men and women with a full complement of teeth, statistically significant sex differences were found among measures related to muscle function and some measures of facial morphology. Thus data from men and women should not be pooled uncritically. The greater bite force in men than women corresponded with the greater diameter and cross-sectional area of type II fibres. Further, the males had more anteriorly inclined mandibles and shorter anterior facial height, suggesting a relation between the greater muscle force and the shape of the face. However, linear regression analysis failed to demonstrate any significant association between bite force and facial morphology among men and women. Thus, craniofacial morphology could be a result of far more contributing factors than previously believed.