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Purpose: Although objective evaluation of articular cartilage is important for assessing the outcome of surgical treatment, no reliable method has yet been developed. It has recently been reported that quantitative ultrasound is applicable for assessment of living human cartilage. The purpose of this study was to investigate whether quantitative ultrasound is able to detect subtle changes in articular cartilage, as well as age-related changes in normal cartilage during arthroscopic surgery. Methods: Thirty-six patients with knee injury underwent ultrasonic evaluation of the articular cartilage during arthroscopy. The reflex echogram from the cartilage was converted to a wavelet map using wavelet transformation. As a quantitative index on the wavelet map, the maximum magnitude was selected. Whether or not the cartilage was damaged was judged from the arthroscopic view of the articular surface. Both normal sites (33 sites) and damaged areas (Outerbridge grade I-II, 11 sites) were measured. Results: The average maximum magnitude values for normal and damaged cartilage were 4.2 ± 1.6 and 1.4 ± 0.6, respectively. The maximum magnitude was significantly higher in intact, than in injured, cartilage ( P < 0.01). The maximum magnitude for intact cartilage of the medial femoral condyle showed a significant correlation with patient age ( r = −0.66, P < 0.01). Conclusions: The present ultrasound measurement system offers potential for the detection of subtle change in cartilage. The maximum magnitude is particularly useful for quantitative assessment of medial femoral condyle articular cartilage. This ultrasound measurement system is useful for diagnosis of degenerative cartilage at an early stage. Level of evidence: Diagnostic, Level III. [ABSTRACT FROM AUTHOR]