Warming patients by changing the ambient environment (rather than core temperature) has been reported to reduce brown adipose tissue activity, thereby reducing artifacts in 18F-FDGPET. Nonetheless, a reduction in cardiac uptake of 18F-FDGhas been incidentally noted during ambient warming. This study examined the impact of seasonal variations in ambient temperatures on cardiac uptake of 18-FDG. Methods: Consecutive 18F-FDGPET patients were recruited into summer and winter cohorts. The protocol was highly standardized; however, data were excluded when scanning took place other than 60 min after injection. Mean and maximum counts per pixel in the cardiac region and cardiac standardized uptake value were determined and correlated with the daily minimum, maximum, and mean temperature (day of scan), mean temperature on the scan day and preceding 2 d (3-d window), and mean temperature 1, 2, 3, and 4 wk before scanning. Results: No statistically significant differences were noted between cohorts (summer and winter) for age, sex, weight, height, or dose. As expected, the summer cohort was associated with statistically higher minimum, maximum, and mean temperatures, longer days, and greater sunlight hours (all P < 0.001). The mean and maximum heart counts were statistically higher in winter than summer (P = 0.031 and P 5 0.024, respectively). The cardiac standardized uptake value was statistically higher for winter than summer (P = 0.026). The key factors in cardiac accumulation of 18FDG were the minimum temperature on the day of the scan and the 3-d mean temperature. Conclusion: Cardiac accumulation of 18F-FDGis influenced by the ambient temperature, in particular the minimum temperature on the day of the scan and the 3-d average temperature before the scan. Further investigation is warranted to examine the impact of this observation on clinical protocols and cardiac 18F-FDGPET study results.