A medical device for localizing a nonpalpable breast lesion. The device includes a tubular introducer needle and a wire guide positioned therein for inserting into a breast to the site of the lesion. The wire guide includes a distal portion having a preformed, 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. The needle is inserted with the wire guide positioned therein into the breast to the site of the lesion and from the distal portion of 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 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 the surgeon to the lesion site during subsequent surgery.