Background: An acute scrotum concerns endoscrotal organs (testicles, spermatic cord, tunica vaginalis) and is characterized by pain, swelling and hyperemia of the hemi-scrotum. It represents one of the most common surgical emergencies in children often caused by testicular torsion; the diagnosis is mostly clinical but must be supported by ultrasonographic examination of the scrotal region in association with a colour Doppler study of the spermatic cord vessels and glandular parenchyma. An idiopathic scrotal hematoma is a very rare condition that can simulate it. Case Report: A 3-day-old full-term baby, otherwise in good health, showed swelling and pain of the left inguinal-scrotal region. A testicular torsion was suspected, so the baby underwent an ultrasound examination of the testis and spermatic cord that showed a left scrotal hematoma with superior displacement of the didymus; the right testicle was located in the internal inguinal canal. Surgical intervention confirmed the sonographic diagnosis of left testicular hematoma and of the right cryptorchidism. Conclusions: Although testicular torsion is the most frequent cause of acute scrotum, the possibility of a persistent idiopathic scrotal haematoma and/or haematoma secondary to a trauma of the inguino-scrotal region, must be always taken into account. US diagnosis can avoid unnecessary emergency surgical treatment, required in case of testicular torsion.