Rupture of simple (extraneural) cysts such as popliteal cysts (Baker's cysts) is a well-known occurrence. The purpose of this report is to introduce the similar occurrence of extraneural rupture of peroneal and tibial intraneural cysts in the knee region, describe the associated magnetic resonance imaging (MRI) findings, and identify risk factors. There was MRI evidence of rupture in 20 of 38 intraneural cases reviewed, mainly in the region of the fibular head and popliteal fossa. Ruptured intraneural cysts and simple cysts share these MRI findings: T2 hyperintense fluid within surrounding intermuscular fascial planes and enhancement with intravenous contrast consistent with inflammation. The mean maximal diameter of the ruptured intraneural cysts was statistically significantly smaller than that of the unruptured cysts. The authors believe that extraneural rupture of an intraneural cyst is due to increased intraarticular pressures transmitted within the cyst and/or elevated extrinsic pressure delivered to the cyst, such as by trauma, akin to the etiology of rupture of extraneural ganglion cysts.