We evaluated the US findings in acute idiopathic scrotal edema (AISE) in order to identify diagnostic features that may help to avoid unnecessary surgical intervention. We investigated 12 boys with AISE diagnosed according to the clinical course and US findings. We assessed the thickness, compressibility, and vascularity of the scrotal wall, the extent of edema, and enlargement of inguinal lymph nodes (LN), and their shape, size, and vascularity. Edematous scrotal wall thickening was observed in all patients. Bilateral thickening was observed in nine patients, and the mean wall thickness was 11.2 mm. Easy compressibility was observed in all patients, and blood flow was increased in 11 patients. Enlargement and hypervascularity of the ipsilateral inguinal LN were observed in all patients. The mean long-axis diameter of the LN was 10.4 mm. The testis and epididymis of patients were normal in most cases. We describe characteristic US findings for AISE, including edema of the scrotal wall with hypervascularity and compressibility, and enlargement of the inguinal LN with hypervascularity. Thus, US may be a useful diagnostic tool to differentiate AISE from other acute diseases of the scrotum.