OBJECTIVE : to compare quantitative kinematic 4-D CT parameters between healthy volunteers and pathologic patients (joint stiffness or chronic ankle instability). METHODS : Thirteen healthy volunteers without previous ankle trauma, 18 patients with clinical subtalar joint stiffness and 10 patients with chronic ankle instability diagnosed based on clinical and imaging findings were prospectively evaluated with 4-D CT. This study was approved by the local ethics committee and all patients signed an informed consent. The subtalar joint was evaluated during a prono-supination cycle. Two angles and two distances between the talus and the calcaneus were measured semi-automatically and independently by two readers. Measurement variations were compared in these three different groups. RESULTS : There were statistically significant differences between axial and coronal talocalcaneal angles of healthy volunteers and patients with joint stiffness (p < 0.0001). The best sensitivities and specificities for the identification of subtalar joint stiffness were 92-100% and 74-94%. Mean and maximal posterior calcaneal facet uncovering were significantly lower in patients with chronic ankle instability patients compared to healthy volunteers (p < 0.006) with sensitivities and specificities of 92-95% and 80-92% respectively. CONCLUSIONS : Quantitative analysis in 4D CT can provide an objective criteria for the differentiation between healthy volunteers and patients with subtalar joint stiffness and chronic ankle instability.