The retention of a foreign body during a surgical procedure or the inadvertent migration of an implant are rare but potentially harmful complications. Especially in the case of the former, the legal situation is unequivocal. Nevertheless, the uncomplicated removal and limitation of the operative trauma may be in the interest of the surgeon. We report and illustrate 2 patients in whom the use of intra-operative ultrasound resulted in the safe and elective removal of an intracranial drill tip in one patient and a contraceptive implant located in the ulnar nerve sheath in the other. The characteristic acoustic shadowing artefact could be demonstrated in both examples. In the first patient, intra-operative magnetic resonance tomography could not be used because of the magnetic artefact, and in the second patient, ultrasound was preferred for reasons of simplicity. Intra-operative ultrasound is an appropriate and easily available tool for detection of small foreign bodies beneath the nervous system. The neurosurgeon should be familiar with the typical characteristics of foreign bodies, which is different from other structures.