Background and Objectives: The objective of this study was to explore the association between 25-hydroxyvitamin D (25(OH)D) and muscle strength/architecture. Methods and study Design: Thirty patients (27 women, 3 men) were allocated into Group I (n=15, mean age; 44.4 +/- 9.4 years) and Group II (n=15, mean age; 39.0 +/- 9.9 years) according to the median of 25(OH)D (<13.7 ng/mL vs >13.7 ng/mL, respectively). Peak torque/body weight of the knee flexor/extensor muscles at 60 degrees/sec and 180 /sec and those of ankle flexor/extensor muscles at 30 degrees/sec and 90 degrees/sec were evaluated by using a Biodex System 3 Pro Multijoint System isokinetic dynamometer. A 7-12 MHz linear array probe was used to evaluate thickness (MT), pennation angle (PA) and fascicle length (FL) of medial gastrocnemius and vastus lateralis muscles. Results: Mean of 25(OH)D was 9.4 +/- 2.5 ng/mL and 20.7 +/- 8.3 ng/mL in Groups I and II, respectively. Although all isokinetic strength parameters were lower in Group I, significant differences were found in knee flexion at 180 degrees/sec (p=0.007), knee extension at 30 degrees/sec (p=0.038) and 180 degrees/sec (p=0.001), and ankle extension at 30 degrees/sec (p=0.002) and 90 degrees/sec (p=0.007). On the other hand, no significant difference was found between the groups regarding MT, PA and FL values (all p>0.05). Conclusion: In light of our results, we can argue that 25(OH)D is associated with muscle strength but not with muscle architecture. Further studies concerning the long-term follow-up effects of 25(OH)D treatment on muscle strength are awaited.