Background: Deep dissecting hematoma is a rapidly extending blood collection that splits the hypodermis from muscle fascia, constituting a medical surgical emergency. The natural history of this condition includes trauma (even minor physical injury) shortly before onset of the lesion, occurring in a patient with advanced dermatoporosis. Case presentation: We report the admission of a 70-year-old woman to the emergency department of our hospital for the onset of a deep dissecting hematoma one month after a negligible trauma in the right leg, complicating secondary iatrogenic dermatoporosis. Bedside ultrasound examination was used to eliminate differential or additional diagnoses and to assess the main features of the hematoma (dimensions, existence of blood supply). Surgical debridement and hematoma drainage were performed due to rapid horizontal extension of the hematoma and unresolved pain, with the operative report confirming the diagnosis. Conclusion: This observation emphasises that in patients with severe dermatoporosis, several weeks can elapse between a minor impact and the sudden development of a limb-threatening deep dissecting hematoma.