Understanding potential determinants of change in television (TV) viewing among children may enhance the effectiveness of programs targeting this behaviour. This study aimed to investigate the contribution of individual, social and home environment factors among 10-year-old Australian children to change in TV viewing over a 21-month period. A total of 164 children (49% boys) completed a 19-lesson (9-month) intervention program to reduce TV viewing time. Children completed self-administered surveys four times over 21 months (pre- and post-intervention, 6- and 12-month follow-up). Baseline factors associated with change in TV viewing during the intervention and follow-up periods were: 'asking parents > or =once/week to switch off the TV and play with them' (21.6 min/day more than those reporting <once/week, p=0.007); being able to 'watch just 1h of TV per day' (26.1 min/day less than those who could not, p=0.010); 'watching TV no matter what was on' (36.6 min/day more than those who did not, p<0.001); and 'continuing to watch TV after their program was over' (33.0 min/day more than those who did not, p=0.006). With every unit increase in baseline frequency of TV viewing with family and friends, children spent on average 4.0 min/day more watching TV over the 21-month period (p=0.047). Baseline number and placement of TVs at home did not predict change in children's TV viewing over the 21 months. Greater understanding of the family dynamics and circumstances, as well as the individual and social determinants of TV viewing, will be required if we are to develop effective strategies for reducing TV viewing in children.