When orbital lesions are discovered incidentally, some have advocated observation given the slow-growing nature of deep dermoids; however, little has been published regarding the true growth potential of these tumors. We describe a case of an incidental deep orbital dermoid cyst in a 7.5-month-old girl with retinoblastoma. The cyst was monitored with five sequential magnetic resonance imaging studies of the orbit over 23 months with careful documentation of growth. The cyst was surgically removed without complication, and histopathology confirmed the diagnosis of dermoid cyst. This case represents the first published quantitative measurement of dermoid growth potential and highlights the need for close early monitoring in incidentally discovered cases.