Self and proxy-reports sometimes are the main source of information on physical activity among schoolchildren. To pool the together, the knowledge on comparability is required. The main aim of the study was to assess the comparability of data gathered from schoolchildren and their mothers concerning physical activity of youths and to identify factors associated with disagreement. A cohort of 91 children (66% girls), aged 7-15 years, and their mothers were studied using a questionnaire in a cross-sectional design. Information gathered included average leisure time physical activity during the summer and winter weeks over one year prior to the interview. The observed level of Lin’s concordance correlation coefficient for agreement between children and their mothers varied from 0.32 for TV/movie viewing to 0.79 for non-seasonal activity. Sedentary lifestyle (>25.5 hours in sedentary activity/week) was identified as a factor associated with disagreement in reports on general seasonal activity (OR=5.47, 95%CI: 1.78-16.83) and on seasonal team games (OR=5.42, 95%CI: 1.74-16.88). Increase in categories of BMI percentiles was associated with an increased risk of disagreement (p for trend 0.037, 0.041; respectively). The comparability of information on the level of children’s and young adolescents’ leisure time physical activity depends on the nature of this activity. Sedentary lifestyle and increased body mass index may be responsible for disagreement in some types of physical activity. The use of parental proxy reporting as a supplementary source of information on the level of physical activity of children should be applied, if ever, with caution.