Dermoid cysts are rare lesions of the CNS with a slow rate of growth. For this reason, they are rarely discovered during infancy. Although benign, these cysts may be associated with devastating complications due to mass effect or meningitis. The discovery of completely asymptomatic dermoid cysts in the pediatric population is exceedingly rare; however, correct and prompt diagnosis is crucial for early surgical treatment to minimize morbidity and mortality. The authors report the unique case of a posterior fossa dermoid cyst discovered in a 5-month-old girl and monitored for 2.5 years with serial imaging studies before performing a resection. The imaging characteristics of dermoid cysts are reviewed, and the challenges associated with the radiographic diagnosis of such lesions are discussed. Analyzing the growth of this particular cyst on MRI allowed comment, for the first time to the authors' knowledge, regarding the growth rate of dermoid cysts. Unlike true tumors, which grow in an exponential pattern, the dermoid cyst in the reported case exhibited a linear growth pattern. The increase in volume followed the classic appearance of a cuboid sequence, which is also consistent with linear growth in all 3 dimensions.