A medical device (10) for localizing a nonpalpable breast lesion (20). The device includes a tubular introducer needle (11) and a wire guide (14) positioned therein for inserting into a breast (25) to the site of the lesion. The wire guide includes a distal portion (16) which is preformed into a resilient helical coil configuration for locking into position about the lesion. The distal portion includes a superelastic metallic alloy for maintaining the helical coil configuration after repeated extensions from and retractions into the needle passageway. With a visualization aid such as an X-ray film or ultrasound, a radiologist typically inserts the needle with the wire guide positioned therein into the breast to the site of the lesion and extends the distal portion of the wire guide from the needle. The distal end of the needle includes a plurality of indentations for enhancing the ultrasound visualization thereof. As the distal portion of the wire guide emerges from the needle, the acuate distal end (17) of the wire guide cuts into and scribes a helical path about the tissue distal to the lesion. The remainder of the distal portion of the wire guide follows the path scribed by the acuate distal tip and locks about the tissue distal to the lesion. Should the needle and wire guide not be appropriately positioned, the distal portion of the wire guide is retracted into the passageway of the needle to reposition the needle and guide. After desired positioning, the needle is removed with the wire guide remaining in a locked position distally about the lesion for guiding guides the surgeon to the lesion site during subsequent surgery.