Watching three or more hours of television a day can double your risk of dying young, say researchers.
Scientists studied 13,284 healthy volunteers with an average age of 37 to investigate links between different kinds of sedentary behaviour and premature death.
Over a period of eight years, they recorded 97 deaths from heart disease, cancer and other causes.
The risk of death was twice as high for participants watching three or more hours of TV a day than for those watching one hour or less.
"Television viewing is a major sedentary behaviour and there is an increasing trend toward all types of sedentary behaviours," said lead researcher Professor Miguel Martinez-Gonzalez, from the University of Navarra in Pamplona, Spain.
"Our findings are consistent with a range of previous studies where time spent watching television was linked to mortality."
No significant association was found between time spent using a computer or driving and higher rates of premature death.
Prof Martinez-Gonzalez added: "As the population ages, sedentary behaviours will become more prevalent, especially watching television, and this poses an additional burden on the increased health problems related to ageing.
"Our findings suggest adults may consider increasing their physical activity, avoid long sedentary periods, and reduce television watching to no longer than one to two hours each day."
The research is reported in the Journal of the American Heart Association.