To evaluate the intra-articular pathology of popliteal cysts of knee, we performed arthroscopy on patients with popliteal cysts between February 1992 and February 1995. Thirty-one patients (32 cases) have been treated by excision of cyst after arthroscopy. Mean duration of follow-up was 15 months (13-19 months). Location of cysts was in the right knee in 17 cases, in the left knee in 13 cases, and bilateral in one case. Pain or swelling was present in 12 (38%), giving-way was reported by 4 cases. Three cases revealed quadryceps atrophy. The McMurray test was postive in 6 cases. After arthroscopy, the cysts were excised. Ninteen cases (59%) had associated intra-articular lesions: seven medial meniscal tears, two lateral meniscal tears, seven had chondral damages, one had medial plica syndrome, and one had rheumatoid arthritis, Communication between the cyst and the knee was present in 7 cases (22%), all of whom were over 40 years of age. Thirteen cases was younger than 30 years of age, and 11 cases of these cases had no intra-articular pathology. Two other cases had lateral meniscal tears and medial plica syndrome, respectively. We recommend arthroscopy in middle and older aged patients with popliteal cysts for evaluating and treating intra-articular lesions.