Stephanie George of the National Cancer Institute and colleagues said two-and-half years after diagnosis, 687 women diagnosed with breast cancer were asked about the amount of time they spent sitting watching television, and the type, duration and frequency of activities they performed in the past year.
They were then followed up for a further seven years, during which time the researchers recorded 89 deaths.
Overall, women who watched the most television were older, more overweight and less active than those who watched the least and more deaths were observed for those who watched the most vs. the least television.
However, once self-reported physical activity levels were taken into account along with other important risk factors, the relationship was no longer significant.
"It is possible that there is no true independent relationship between post-diagnosis television time and death. The breast cancer survivors who reported the most television time also reported the equivalent of 140 minutes per week of moderate-to-vigorous intensity physical activity, which is the amount recommended to all adults for general health," George said in a statement.
"Perhaps with this amount of recreational activity, television time may not have an independent effect on survival."
The study, published in the Journal of Cancer Survivorship, said after accounting for self-reported physical activity levels after diagnosis, sedentary behavior was not an independent risk factor for death.