OBJECTIVE The purpose of this study was twofold: (1) to determine the characteristic MR appearance of the xanthomatous tendons of heterozygous familial hypercholesterolemia and (2) to determine which of two imaging techniques, high-frequency linear-array sonography or MR imaging, is better for detection of xanthomas. METHODS Sonography and MR imaging were performed to evaluate the Achilles tendons in 10 patients (20 tendons) with heterozygous familial hypercholesterolemia. For sonography, 7.5- (nine patients) and 10.0- (one patient) MHz transducers were used. T1-weighted, proton density-weighted, and T2-weighted MR images were obtained at 0.5 T (four patients) and 1.5 T (six patients), including fat-suppressed (six patients) and water-suppressed (one patient) T1-weighted images at 1.5 T. Tendon abnormalities detected with both techniques were noted, and the results were compared. RESULTS In all tendons, sonograms showed focal hypoechoic lesions compatible with xanthomas. MR images did not show focal lesions. Instead, all pulse sequences showed a diffuse speckled or reticulated pattern or both on axial images. This speckled or reticulated appearance was more obvious on fat-suppressed T1-weighted images and much less evident on water-suppressed T1-weighted images. Contrast resolution was subjectively better on sonograms than on MR images in all cases. CONCLUSIONS The speckled or reticulated appearance is a characteristic, if not pathognomonic, MR feature of xanthomatous tendons and probably is due to edema or inflammation, not intratendinous lipid. However, localized lipid deposits detected on sonograms are more readily quantified than are the lesions seen on MR images. Therefore, we think sonography, rather than MR imaging as performed in our study, is the technique of choice for detecting xanthomas.