Summary Background  Liver stiffness measurement (LSM) using transient elastography (TE) is used to stage fibrosis in patients with liver disease, diagnostic reliability and the factors affecting its performance in patients with non‐alcoholic fatty liver disease (NAFLD) are incompletely understood. Aim  To assess LSM. Methods  Consecutive NAFLD patients (n = 169), assessed by liver biopsy (Kleiner score), anthropometrical, biochemical and metabolic features, underwent LSM using TE with standard M probe. Results  Liver stiffness measurement was not reliable in 23 patients (14%) due to obesity. Among patients with a reliable TE, a LSM value >7.25 kPa was the best cut‐off for predicting significant fibrosis at biopsy (AUC 0.794); however, this cut‐off still failed to rule out F2‐F4 fibrosis in 31% (false‐negative rate) or rule in F3‐F4 in 29% (false‐positive rate). Similarly a LSM value >8.75 kPa was the best cut‐off for severe fibrosis (F3‐F4) (AUC 0.870), with a rate of false‐negatives 24% and of false‐positives 2%. Body mass index was the major determinant of these diagnostic errors in predicting significant and severe fibrosis both by overestimating or underestimating the stage of fibrosis. Conclusions  In NAFLD patients, even when liver stiffness measurement is feasible, high BMI values negatively affect the diagnostic reliability. Improved performance of transient elastography could be obtained using specifically designed probes.