A primary peripheral cyst behind the iris is difficult to detect during a routine slit lamp examination. It is usually asymptomatic unless the cyst is sufficiently large. We report on a 73-year-old woman who underwent phacoemulsification of a cataract and incidentally presented a small dark mass behind the peripheral iris. The lesion was initially suspected of being a melanoma, but it could not be localized by conventional techniques postoperatively. A diagnosis of primary iris cyst was not made until 1 year later when high-frequency ultrasound biomicroscopy (UBM) was employed. UBM displayed a distinct thin-wall cystic lesion (0.5 x 0.8 x 0.6 mm) with low internal reflectivity at the iridociliary sulcus of the inferotemporal quadrant. UBM allows good imaging access to a lesion behind the iris that is otherwise difficult to detect with traditional techniques.