OBJECTIVE To estimate the safety window for needle insertion in the posterior approach to the tibialis posterior by ultrasonography, particularly in a clinical setting. METHODS Cross-sectional study. METHODS University hospital. METHODS Healthy volunteers (108 legs, from 22 men and 32 women). METHODS Not applicable. METHODS Distance between the tibia and neurovascular bundles in the posterior aspect of the tibia on a transverse ultrasonographic scan. RESULTS The safety window at the midpoint was significantly larger than the upper third (0.62-2.16 cm, average 1.47 ± 0.38 cm vs 0.51-1.62 cm, average 1.16 ± 0.31 cm). The depth at the midpoint was significantly more shallow than the upper third (1.57-3.16 cm, average 2.31 ± 0.34 cm vs 1.76-3.66 cm, average 2.52 ± 0.38 cm). Body weight, height, tibial length, and leg circumference showed positive correlation with the safety window at both points. CONCLUSIONS The midpoint may be more favorable than the upper third for needle insertion to the tibialis posterior with a posterior approach.