Arachnoid cysts are relatively common lesions encountered in neurosurgical practice. While arachnoid cysts are most commonly observed to remain of fixed volume over time, little is actually known about their natural history and optimal method of treatment. We present 2 young children with middle fossa arachnoid cysts. In 1 child, the cyst enlarged over time, prompting neurosurgical intervention. However, in the other child, the cyst resolved spontaneously without intervention. The child whose cyst enlarged underwent cyst wall fenestration and cyst-peritoneal shunting. For the child whose arachnoid cyst disappeared without treatment, there was no history of trauma or antecedent headache. These 2 cases serve to illustrate in a single report that middle fossa arachnoid cysts can be dynamic in nature, with some growing in size while others spontaneously resolving. In both cases presented, the temporal lobe on the affected side was compressed by the large cyst. Following treatment or spontaneous resolution of the cyst, the temporal lobe reexpanded, suggesting a normal volume of temporal lobe, and not temporal lobe agenesis. In asymptomatic patients with moderate-sized arachnoid cysts such as the ones demonstrated in these case reports, we suggest close follow-up with serial imaging before deciding on a final treatment plan.