A high frequency sonographic technique-ultrasound backscatter microscopy-was used to visualize the subsurface structure of immature porcine articular cartilage from the knee joint. In 20-week-old pigs, all parts that were scanned, except the weight-bearing regions of the femoral condyles, demonstrated heterogeneous ultrasound backscatter characteristics within the articular cartilage. A trilaminar pattern consisting of hypoechoic, hyperechoic, and anechoic layers ranging from superficial to deep generally was observed, except in the weight-bearing regions of the femoral condyles, where a homogeneous anechoic pattern was seen. In the younger pigs (6 and 10 weeks old), the trilaminar backscatter pattern was not observed. Small, highly echogenic structures that correlated with vascular channels in histologic assessment were observed frequently in the cartilage of younger pigs, but they were seldom present in the cartilage of 20-week-old pigs. Structural details, such as disruption of the subchondral bone and presence of a thickened fibrous layer on the articular surface at the chondrosynovial junction, also were detected with the ultrasound backscatter microscope. We concluded that high frequency ultrasound can be used to visualize the subsurface structure of immature articular cartilage and some of its developmental changes. Further research is required to explain the mechanism underlying the observed backscatter characteristics of immature articular cartilage and to study its potential for the imaging of pathologic changes.