A 21-year-old patient with metastatic osteosarcoma was receiving methotrexate with leucovorin rescue every 2 weeks. After the second (of four) infusions of methotrexate, a prior solar burn on an area of skin was reactivated in spite of leucovorin rescue. An area of skin treated 5 months previously by radiation was spared the effects of the reactivation phenomenon. N o other toxicities appeared.T h e reactivation of the solar burn is a n example of "false photosensitization" and this cutaneous toxicity is not ameliorated by leucovorin. Methotrexate therapy should be delayed until the effects of generalized solar burns have resolved (approximately 1 week). A recent review' of the dermatologic toxicities of antineoplastic agents mentions that methotrexate is associated with reactivation of radiation dermatitis; however, no mention was made of reactivation of solar burns with methotrexate. Dermatitis is commonly described as a toxic effect of methotrexate but in most reports the character of the dermatitis is not specified or it is described as one of the following: (a) reactivation of a radiation dermatitis; (b) generalized maculopapular eruptions occasionally progressing t o bullous eruptions associated with other manifestations of methotrexate toxicity such as stomatitis, bone