Introduction: One of the factors that contribute to the total needle-targeting error in medical procedures is needle deflection, which can lead to e.g. hemorrhage, prolonged intervention time and decreased treatment efficiency. This deflection can be defined as the deviation of the needle from its suspected straight insertion path. In this study, we focused on the effect of tissue heterogeneity on needle deflection. We hypothesized that needle deflection would be bigger for insertions into heterogeneous than homogeneous specimens, due to an unequal force distribution on the needle. Method: the inner needle of a 18Gauge trocar needle with triangular tip was inserted (5mm/s) multiple times at several positions into 4 gelatin-, 4 animal liver-, and 2 human liver specimens. Deflection in X- and Y-direction was measured using sliding gauges and axial forces acting on the needle were captured using a force sensor mounted onto the needle hub. Axial force analysis was used to give a rough estimation of the mechanical properties of the specimens. Results: Results show an increase in magnitude and variance for needle insertions into the liver specimens (Animal: Mean = 1.01mm, SD = 0.54, Human: Mean = 0.83mm, SD = 0.48) compared with those into gelatin specimens (Mean = 0.59mm, SD = 0.26mm). Differences between the median and maximal needle tip forces for insertions into gelatin were almost zero (Mean = 0.08, SD = 0.04N), whereas those differences were bigger for insertions into tissue specimens (Mean = 0.58, SD = 0.23N), indicating the homogeneous nature of gelatin and heterogeneous nature of tissue. Discussion: The results obtained in this study suggest that heterogeneity causes the needle to deflect from its straight path. Both magnitude and variance of needle deflection are bigger for insertions into heterogeneous tissue specimens than into homogeneous gelatin specimens. A suggestion for future research is to study the effect of pathologic tissue on needle deflection, as this type of tissue is known for being more heterogeneous than healthy tissue.