To determine whether ATP and P2X3 receptors contribute to bone-cancer pain in a mouse model, immunohistochemical techniques were used to identify whether changes in the labeling of P2X3 receptors on epidermal nerve fibers (ENFs) occurred during tumor development. C3H mice were injected with osteolytic fibrosarcoma cells in and around the calcaneus bone. These mice exhibited mechanical hyperalgesia by day 10 post-implantation as assessed using von Frey monofilaments. Biopsies of the plantar skin overlying the tumor were obtained at days 10, 14, and 18 post-implantation. Confocal images were analyzed for the number of PGP 9.5, P2X3, and CGRP immunoreactive (ir) ENFs. The overall ENF population (PGP-ir) decreased progressively over time, whereas the subsets of P2X3-ir fibers demonstrated a modest increase and CGRP-ir nerve fibers remained fairly constant. Importantly, the proportion of CGRP-ir fibers that labeled for P2X3 increased from approximately 6% in control animals to nearly 30% at day 14 following tumor cell implantation. These studies demonstrate increased expression of P2X3 receptors on CGRP-ir ENFs during tumor growth and suggest a role for ATP in cancer-related pain.