YKL-40 concentrations in serum were determined by an ELISA at 3 occasions during 19 months for 57 early RA patients. The results were related to biochemical and radiographic measures at each time point. YKL-40 correlated significantly to ESR and CRP throughout the study. Correlations between YKL-40 and radiographic findings scored by the Larsen method were fairly weak both for absolute values at each time point (Rs 0.212-0.319) and for progression over time (Rs 0.152-0.301). Baseline YKL-40 could predict radiographic progression with a specificity and sensitivity of only slightly over 50%. ESR and CRP correlated stronger than YKL-40 to joint damage progression and in a multiple regression model ESR was the only significant variable explaining the variance of this radiographic measure. We conclude that serial measurements of serum YKL-40 did not provide information that could not be obtained by conventional biochemical measures of disease activity.