$Purpose:$ Leisure-screen-time, including TV viewing, is associated with increased mortality risk. We estimated the all-cause mortality risk reductions associated with substituting leisure-screen-time with different discretionary physical activity types, and the change in mortality incidence associated with different substitution scenarios.

$Methods:$ 423,659 UK Biobank participants, without stroke, myocardial infarction or cancer history, were followed for 7.6 (1.4) (median (IQR)) years. They reported leisure-screen-time (TV watching and home computer use) and leisure/home activities, categorised as daily-life activities (walking for pleasure; light DIY; heavy DIY) and structured exercise (strenuous sports; other exercises). Iso-temporal substitution modelling in Cox regression provided hazard ratios (95% confidence intervals) for all-cause mortality when substituting screen-time (30 minutes/day) with different discretionary activity types of the same duration. Potential impact fractions (PIFs) estimated the proportional change in mortality incidence associated with different substitution scenarios.

$Results:$ During 3,202,105 person-years of follow-up, 8,928 participants died. Each 30 minute/day difference in screen-time was associated with lower mortality hazard when modelling substitution of screen-time by an equal amount of daily-life activities (0.95 (0.94-0.97)), as well as structured exercise (0.87 (0.84-0.90)). Re-allocations from screen-time into specific activity subtypes suggested different reductions in mortality hazard (walking for pleasure (0.95 (0.92-0.98)), light DIY (0.97 (0.94-1.00)), heavy DIY (0.93 (0.90-0.96)), strenuous sports (0.87 (0.79-0.95)), other exercises (0.88 (0.84-0.91))). The lowest hazard estimates were found when modelling replacement of TV viewing. PIFs ranged from 4.3% (30 minute/day substitution of screen-time into light DIY) to 14.9% (TV viewing into strenuous sports).

$Conclusion:$ Substantial public health benefits could be gained by replacing small amounts of screen-time with daily-life activities and structured exercise. Daily-life activities may provide feasible screen-time alternatives, if structured exercise is initially too ambitious.