The main purpose of the study is to investigate the inversion/ eversion muscle strength balance of the ankle in patients with medial tibial stress syndrome (MTSS). A dysbalance of these muscles may play a role in the pathophysiology of MTSS. Another aim is to measure the medial longitudinal arch and navicular drop in patients with MTSS. A total of 11 patients diagnosed with MTSS in the outpatient clinic of Ege University School of Medicine Sports Medicine Department were enrolled in this study. The control group consisted of 11 regularly exercising individuals. The mean age of the patient group and the control group was 21.0 +/- 1.9 years (18-23 years) and 23.2 +/- 2.9 years (18-27 years), respectively. A detailed exercise questionnaire was administered to all subjects. Isokinetic muscle strength testing was performed at 30 degrees/sec and 120 degrees/sec to assess invertor and evertor muscle strength of the ankle. Photographs of the weight bearing and non-weight bearing foot were taken to measure the medial longitudinal arch deformation and the navicular drop. At 30 degrees/sec, the average eversion concentric strength was significantly higher in the patient group, and the inversion/eversion strength ratio was significantly higher in the control group (p < 0.05). At 120 degrees/sec velocity, average concentric eversion strength was significantly higher in the patient group (p < 0.05). The measurements of pronation indicators did not reveal any statistically significant differences between the two groups (p > 0.05). MTSS may occur without an increase of pronation indicators like medial longitudinal arch deformation or navicular drop. In such cases, one of the predisposing factors may be the strength dysbalance of the invertor/evertor muscles in favour of the evertor muscles. This observation may be of additional value in the prevention and therapy of MTSS.