Objective Patients diagnosed with psoas abscess have a high mortality rate. The major cause of its poor prognosis is delayed treatment. Therefore, making a correct diagnosis rapidly is important. Both computed tomography (CT) and magnetic resonance imaging (MRI) are considered to be the gold standards as imaging modalities that have a high sensitivity for detecting psoas abscess. There have been few reports regarding the limitations of these methods, but psoas abscess in its early stage may go undetected by CT and MRI. Methods Detection of psoas abscess by CT and MRI was investigated in the present study through a retrospective review of 15 patients in whom psoas abscess was diagnosed during a course of ten years at our hospital. Results In all patients, psoas abscess was diagnosed by at least a plain CT, enhanced CT, and/or plain MRI. The interval between the onset of symptoms and diagnosis was 20.9±17.9 days (mean ± standard deviation). In three patients, repeat imaging identified a psoas abscess, whereas initial imaging failed to detect it. The overall sensitivity of plain CT, enhanced CT, and plain MRI for psoas abscess was 78%, 86%, and 88%, respectively. From six days after the onset of symptoms, the sensitivity of each modality was 100%, while the sensitivity from day one to five days was only 33%, 50%, and 50%, respectively. Conclusion Although CT and MRI are considered to be gold standard modalities for diagnosing psoas abscess, both methods can fail to notice this condition in its early stage.