A 67-year-old Japanese man visited our hospital complaining of back pain and was diagnosed with squamous cell lung carcinoma (cT1aN3M1b: IV) by CT-guided needle biopsy from a metastatic lesion in the longissimus muscle. Before chemotherapy, he presented shortness of breath and palpitation and was admitted to our hospital. Laboratory data showed acute renal failure, hyperuricemia, hyperkalemia, hyperphosphatemia, hypocalcemia, and disseminated intravascular coagulation (DIC). Electrocardiogram showed atrial fibrillation. These data matched the criteria for clinical and laboratory tumor lysis syndrome (TLS), and he was diagnosed as having spontaneous TLS and DIC complicated with squamous cell lung carcinoma. The patient died as a result of multiple organ failure 34 hours after admission. Our case and summary suggest that lung cancers can rarely be complicated with spontaneous TLS irrespective of their histology, and hyperphosphatemia and time to a diagnosis of TLS are risk factors for a poor prognosis of TLS. Therefore spontaneous TLS should be considered aggressively when hyperuricemia and abnormal electrolytic imbalance are found in lung cancer patients, even before treatment with chemotherapy and radiation therapy.