Purpose: We evaluated 18F-FDG PET/CT findings in initially diagnosed adenoid cystic carcinoma (ACC) of the head and neck in association with pathological subtype, staging, uptake comparison with squamous cell carcinoma (SqCC) and prognosis. Materials and Methods: The subjects were 16 patients with initially diagnosed ACC of head and neck who underwent pretreatment 18F-FDG PET/CT. Histological subtype (solid pattern vs. tubular/cribriform pattern), SUVmax of size-matched SqCC of the head and neck as control group, disease-free survival (DFS) were compared with the SUVmax of ACC of the head and neck. Results: Of total 16 patients, 6 had solid pattern and the remaining 10 had tubular/cribriform pattern. The SUVmax were significantly higher in solid pattern group than in tubular/cribriform pattern group (6.7±3.2 vs. 4.2±0.9, p=0.03). PET/CT found unexpected distant metastasis in 18.7% of patients (3/16) and changed the therapeutic plan in those patients. The SUVmax of ACC was significantly lower than that of size-matched SqCC (5.1±2.4 vs. 13.6±6.0, p<0.001). DFS was not significantly different according to the histological subtype. In contrast, patients with high 18F-FDG uptake (SUVmax ≥6.0) had significantly shorter DFS than those with low 18F-FDG uptake (SUVmax <6.0). Conclusion: 18F-FDG uptake of ACC of the head and neck is significantly associated with histological subtype and DFS. 18F-FDG PET/CT may be useful for detecting unexpected metastasis. Since 18F-FDG uptake of tubular/cribriform ACC compared with SqCC is relatively low, it is necessary to interpret PET images carefully in patients without alleged ACC.