Valid and simple instruments to assess physical activity (PA) in specific populations are required for health-related research. The aim of the present study was to assess the validity of the Bouchard activity diary (AD) in Spanish adolescents using an activity monitor to compare total PA and moderate-to-vigorous PA (MVPA) obtained by both instruments.

Sixty-one Spanish adolescents, aged 12–16 years, completed the Bouchard AD and wore the ActiGraph activity monitor for three consecutive days. Validity was assessed with the Spearman correlation coefficient (ρ), the Bland–Altman method and the κ coefficient.

Thirty-seven adolescents were included in the final analysis. Correlations between the activity monitor and the AD administered over the three days (Thursday–Saturday) were moderate (ρ = 0·33–0·35, P < 0·05) or non-significant for total PA and moderate (ρ = 0·36, P < 0·05) for MVPA. Correlations between the two methods were progressively lower for each subsequent day of testing, for both total PA and MVPA. The Bland–Altman plot illustrated that the Bouchard AD overestimated MVPA (mean difference −32·05 (sd 74·56) min; 95 % limits of agreement 109·61, −173·31 min). Agreements for classification into MVPA tertiles and accordance with the international recommendations of MVPA were fair and moderate, respectively, for the 3 d means.

The Bouchard AD has reasonable validity to assess total PA and MVPA in Spanish adolescents. The results show lower levels of agreement on the third day but it is not clear if this is due to design features (weekday v. weekend) or to participant compliance with the survey or the activity monitoring protocol.