Ectopic gas in the graft is occasionally encountered upon follow-up computed tomography (CT) after anterior cervical corpectomy and fusion (ACCF). However, most cases lack inflammatory responses and manifestations of infection. We reviewed 112 patients who underwent ACCF and follow-up CT, with a minimum follow-up period of 2 years. CT images were retrospectively reviewed to confirm the presence of ectopic gas in the graft and bone fusion. Bone fusion was defined as follows: mobility less than 2 mm between spinous processes on the flection-extension radiograph or a bone bridge on CT images. Of the 112 patients, 30 (27%) patients had ectopic gas in the fibular grafts. Among them, ectopic gas was initially observed 3 months after surgery (early onset) in 23 (77%) patients and 6 months after surgery (late onset) in the remaining seven (23%) patients. Upon the latest follow-up CT, ectopic gas more frequently remained in late-onset (4/7, 57%) rather than in early-onset (3/23, 13%) cases (p = 0.033). Bone fusion was not observed when CT images exhibited ectopic gas in the graft, whereas ectopic gas was not observed when CT images exhibited bone fusion. Late-onset gas tended to significantly remain. The remaining gas was strongly associated with pseudoarthrosis.