We illustrate the case of a brachial pseudoaneurysm in a 32-week preterm baby male who presented with a large pulsatile mass in the right antecubital fossa, with no clear aetiologic factor. The management of this type of lesion has been controversial and based mainly on case reports. In this case, after obtaining partial response with two thrombin injections, it spontaneously regressed during expectant management. This combination of therapeutic options may be an alternative for the management of complex lesions and, to the best of our knowledge, has not been previously reported.