The purpose of this study was to determine the prevalence of peripheral rim instability in discoid lateral meniscus. A consecutive series of 112 patients (128 knees) (mean age 10.0 years [range 1 month to 22 years]) who underwent arthroscopic evaluation and treatment of a discoid lateral meniscus between 1993 and 2001 was reviewed. Of those discoid menisci classified intraoperatively (n = 87), 62.1% (n = 54) were complete discoid lateral menisci and 37.9% (n = 33) were incomplete discoid lateral menisci. An associated meniscal tear was present in 69.5% (n = 89) of all knees studied. Overall, 28.1% (n = 36) of discoid lateral menisci had peripheral rim instability: 47.2% (n = 17) were unstable at the anterior-third peripheral attachment, 11.1% (n = 4) at the middle-third peripheral attachment, and 38.9% (n = 14) at the posterior-third peripheral attachment. Thirty-one of the 36 unstable discoid menisci underwent repair of the peripheral meniscal rim attachment. One patient underwent a complete, open meniscectomy. Peripheral rim instability was significantly more common in complete discoid lateral menisci (38.9% vs. 18.2%; P = 0.043) and in younger patients (8.2 vs. 10.7 years; P = 0.002). The frequency of peripheral instability mandates a thorough assessment of meniscal stability at all peripheral attachments during the arthroscopic evaluation and treatment of discoid lateral meniscus, particularly in complete variants and in younger children.