Unilateral congenital vitreous cysts (VC) were observed in two young patients. The vitreous was highly liquefied in both. Biomicroscopially, the ball-shaped VC in the first patient was seen to be detached from the surrounding tissues and hence highly mobile. The potato-shaped VC in the second patient was attached to the posterior lens surface by a short, thread-like strand and was therefore only slightly mobile. Partially vascularized prepapillary strands were observed in the patient's fellow eye. Two different pathogenic mechanisms may be postulated, namely (1) that the VC was squeezed or jarred loose from the ciliary body pigment epithelium (first patient), or (2) that the VC resulted from impaired retrogression of the primary vitreous or the hyaloid artery (second patient).