Collision (contiguous) tumors of the skin can result in misleading clinicopathological presentations, and the choice of appropriate diagnostic techniques may prevent incomplete diagnosis and management. We report 2 cases of collision tumors involving amelanotic malignant melanoma of the back. One patient is a 79-yr-old male with an 8.7 x 5.5 x 4.5 cm polypoid lesion that on shave biopsy was diagnosed as basal cell carcinoma. Subsequent excision showed that the lesion was largely composed of amelanotic melanoma underlying a relatively small and thin basal cell carcinoma, and this probably would have been demonstrated in a punch (rather than shave) biopsy. The other patient is a 71-yr-old male with a 1 cm exophytic lesion on the back, which was determined microscopically to be melanoma, and a 0.6 cm papule on the back. This lesion was composed of 2 distinct contiguous neoplastic infiltrates, the predominant component being an atypical fibroxanthoma and the smaller component an amelanotic melanoma (primary vs metastatic), with diagnostic confirmation requiring multiple immunohistochemical stains.