PURPOSE: Within Hoffa's infrapatellar fat pad there are two synovial clefts, horizontal and vertical, whichcommunicate with the intra-articular space. Intra-articular lesions can also occur in these clefts, and are oftendifficult to differentiate from extra-articular lesions. The purpose of this study is to evaluate, using MRimaging, the lesions occurring in these synovial lined clefts, as well as associated abnormalities. MATERIALS AND METHODS: Thirty-one knees of 31 patients with lesions in horizontal and vertical clefts in Hoffa's infrapatellarfat pad were retrospectively evaluated. Using a 1.5T MR imager, axial, sagittal, and coronal MR images of kneeswere obtained. Lesions in clefts, degree of joint effusion and associated knee abnormalities were analyzed. RESULTS: Horizontal cleft lesions were noted in 21 cases ; there were 17 cystic dilatations, two loose bodies,one synovial chondroma, and one case of pigmented villonodular synovitis(PVNS). Vertical cleft lesions were notedin 15 cases ; these comprised 11 cystic dilatations, two loose bodies, one synovial osteochondromatosis, and onePVNS. Among all cases, three cystic dilatations, one loose body, and one PVNS occurred in both horizontal andvertical clefts. Among 25 knees with cystic dilatations of clefts, five showed grade I joint effusion, ten gradeII, and ten grade III. Associated abnormalities were meniscal tear in 16 cases, osteoarthritis in 13, cruciateligament tear in five, osteochondritis dissecans in three, osteochondral fracture in two, osteonecrosis in one,loose body in one, and synovitis in one. CONCLUSION: Among synovial-lined clefts in Hoffa's fat pad, the mostcommon lesion was cystic dilatation ; there were various associated abnormalities and a close relationship tojoint effusion. An awareness of the types of lesions found in clefts is helpful for narrowing the differentialdiagnosis of lesions occurring in the area of Hoffa's fat pad.