Purpose: This is a retrospective observational study that assessed the prevalence of positive diagnostic imaging and electrodiagnostic (EDX) findings in patients diagnosed with pronator syndrome (PS), who previously had a carpal tunnel syndrome (CTS) surgery. The other purpose of our study was to determine how often PS occurred and was missed in patients treated surgically for CTS. Methods: The files of 180 patients who underwent CTS surgery were reviewed retrospectively. We assessed all patients for a diagnosis of PS. We accepted the clinical findings and patient history as the reference standard for the diagnosis of PS. Anteroposterior and lateral radiographs of the elbow, bilateral upper limb magnetic resonance imaging (MRI) studies, and bilateral dynamic forearm ultrasound (US) were performed on patients with clinical symptoms and physical examinations that indicated PS. Bilateral upper limb EDX was also performed for these patients. One patient refused additional tests. Results: A total of 174 extremities in 146 patients were included in the study. Pronator syndrome was diagnosed by 2 hand surgeons in 22 extremities (19 patients) through a clinical evaluation that included a history and physical examination. Diagnostic testing was positive for findings of PS in 24% of extremities (5 of 21) tested by EDX, in 57% of extremities (12 of 21) tested by US, and 5% of extremities (1 of 21) tested by MRI. There was no lower humeral spur that could cause median nerve compression on any plain radiographs. Conclusions: With clinical evaluation as the reference standard, EDX, US, and MRI are not helpful in making a diagnosis of PS concurrent with CTS. Type of study/level of evidence: Diagnostic IV.