Brown adipose tissue (BAT) has been widely reported to affect the efficacy of 18F-FDG PET in oncology. Several small studies have found that warming the patient both before 18F-FDG injection and during the uptake phase can reduce the frequency of BAT uptake. Pharmaceutical approaches such as propranolol and diazepam have also been used. Methods: We describe two case studies that demonstrated the impact of combined patient warming and diazepam administration on BAT uptake in pediatric 18F-FDG PET images. To support these findings, we performed a retrospective analysis of 139 patients: 71 who received warming only and 68 who received warming combined with diazepam. Results: BAT uptake was significantly less frequent in patients who received diazepam (16.2%) than in those who did not (33.8%) (P = 0.0167), particularly in female patients (7.4% vs. 26.7%) (P = 0.0486). Conclusion: Although patient warming alone reduces the frequency of BAT uptake, the addition of diazepam to patient warming significantly furthers the reduction in young patients, particularly female patients.