Background: Regular physical activity is important for remaining healthy. Most studies on the association between active child- and adulthood were based on small non-representative populations. The purpose of the study was to quantify tracking of leisure-time PA (in and outside sports clubs) for 6 years from adolescence into young adulthood in a representative sample in Germany. Methods: This study was a subsample of the "Motorik-Modul (MoMo) Longitudinal Study" (baseline: 2003-2006, wave 1: 2009-2012). Representative longitudinal physical activity data of N∈=∈947 adolescents were included and collected using the MoMo-physical activity questionnaire (MoMo-PAQ). Stability of different physical activity indices was measured using Spearman's rank-order correlations and ANOVA with repeated measurement with age, sex and socio-economic status (SES) as determinants. Results: While mean leisure-time physical activity outside sports clubs (LTPA) (F<inf>1,397</inf>∈=∈7.9, df∈=∈1; p∈<∈.01), sports club physical activity (SCPA) (F<inf>387</inf>∈=∈4.8, df∈=∈1; p∈<∈.05) and overall physical activity (OPA) (F<inf>1,441</inf>∈=∈7.7, df∈=∈1; p∈<∈.01) changed significantly over time, no changes in overall sports index (OS index) (F<inf>371</inf>∈=∈3.2, df∈=∈1; p∈>∈.05) were observed. ... mehrLow tracking correlations were found for different physical activity indices (LTPA: r∈=∈.094, p∈<∈.05; SCPA: r∈=∈.248 p∈=∈<.05; OPA: r∈=∈.211 p∈<∈.05; OS index: r∈=∈.266 p∈<∈.05). Results by sex, age and SES were inconsistent. Analyses of agreement showed different results for determinants and settings. Conclusion: The results of this representative study were comparable to previous studies and showed significant but low stability. Possible reasons for low stability coefficients are a relatively long timespan between both measurement points and potential effects of the reliability of subjective assessment methods. The results confirm that physical activity is a fluctuating variable. Future studies should examine the determinants of tracking and change in physical activity. © 2015 Rauner et al.; licensee Biomed Central.