Metastatic bone tumors occasionally are found in association with a "sunburst" periosteal proliferation of bone resembling that of primary bone tumors. Five such cases are reported and discussed, including 2 retinoblastomas, 1 prostatic carcinoma, 1 tumor of undetermined primary origin (probably bronchogenic) and 1 chloromatous acute myelocytic leukemia. It is concluded from the cases reported that the nature of the "sunburst" reaction is independent of the specific histology of the metastatic tumor and is essentially a response of the involved bone. It is noted that the "sunburst" pattern tends to occur with rapidly growing tumors in which there is both bone and extraosseous involvement and that the response occurs near, but not immediately adjacent to, destructive tumor foci. Although the exact manner of production of the "sunburst" pattern is unknown, it is suggested that bioelectric potentials generated by tumor cells and tumor injury may well play a role.