Solar burn reactivation, a rare and idiosyncratic drug reaction, has been reported with the use of a variety of drugs. This reaction is believed to be the result of exposure to ultraviolet light during the subsiding phase of an acute inflammatory reaction. It affects areas of the body that have been previously sunburned. We describe a 16-year-old girl who was receiving treatment for acute lymphoblastic leukemia and experienced a second-degree solar burn reactivation reaction to methotrexate. The patient had a mild sunburn on her face and shoulders the day she went to the oncology clinic for her interim maintenance chemotherapy with vincristine 1.5 mg/m(2)/dose and methotrexate 100 mg/m(2)/dose. Three days later, she returned to the clinic with a 2-day history of fever (<or= 100.2 degrees F), nausea, vomiting, and malaise; the sunburn on her face and shoulders also had become severe, without further sun exposure. Laboratory results revealed elevated blood urea nitrogen and serum creatinine concentrations, and her methotrexate level was elevated at 0.9 mM. The patient was diagnosed with acute renal failure, dehydration, methotrexate toxicity, and second-degree solar burn reactivation reaction. She was admitted to the children's hospital and treated with sodium bicarbonate, acetaminophen with codeine, ondansetron, and silvadene cream. On hospital day 3, the patient's methotrexate level decreased to less than 0.1 mM. The sunburn continued to heal, and after a 14-day hospital stay, complicated by a streptococcal infection, grade 3 mucositis, bacteremia, and mild gastritis and duodenitis, the patient recovered and was discharged. Use of the Naranjo adverse drug reaction probability scale indicated a probable relationship (score of 6) between the patient's solar burn reactivation and methotrexate. Although methotrexate-induced solar burn reactivation is rare, clinicians should be aware of this potential adverse reaction and consider delaying administration of methotrexate by 5-7 days if a patient reports ultraviolet-related erythema in the past 2-4 days or presents with a notable sunburn.