Introduction Ultrasound (US) evaluation of peripheral nerves is a noninvasive, cost‐effective approach to diagnosing focal mononeuropathies and guiding surgical management. We used the intranerve ratio to evaluate for possible cut‐off values in diagnosis of fibular mononeuropathies (FNs). Methods A retrospective analysis of FN confirmed by electrodiagnosis (EDx) was performed to identify intranerve ratio values between affected and unaffected limbs at the fibular head and popliteal fossa. Results The optimal fibular head/popliteal fossa intranerve ratio to discriminate between limbs with and without disease was 1.25 (sensitivity, 51%; specificity, 71%). There was no statistically significant difference between affected vs unaffected limbs (ratio, 1.13; P = .15) nor in subgroup analyses. However, 25% of patients had structural lesions amenable to surgery. Discussion The utility of US in diagnosis of FN is limited using intranerve ratio data, but US has a distinct advantage over EDx for identifying treatable structural lesions.