BACKGROUND: Hypoglycemia due to non-insulin-producing tumors is referred to as non-islet cell tumor hypoglycemia (NICTH). As NICTH is a rare lesion, the natural course of NICTH is not well understood. We report a case of NICTH that was observed 30 years before the onset of hypoglycemia. CASE SUMMARY: A 50-year-old man was diagnosed with an abnormal right chest shadow during a routine X-ray examination, but no further examination was undertaken because the lesion appeared benign. Thirty years after the tumor discovery, the patient was admitted to the hospital with symptoms of severe hypoglycemia, which was diagnosed as NICTH based on a complete examination. The tumor was resected and found to be a solitary fibrous mass (15.6 cm × 13.7 cm × 10.4 cm); thereafter, the patient's blood glucose levels normalized and he completely recovered. CONCLUSION: NICTH can have an acute onset, even if the tumor has been present and asymptomatic over a long time period.