MACROSCOPIC BENIGN PINEAL cysts, according to previous reports, may preferentially occur in the young population at the fourth decade; symptomatic pineal cysts have been found mainly in females. To confirm the relationship of age and sex to the incidence of pineal cysts, we reviewed 8623 consecutive magnetic resonance images in 6023 patients. Tiny and obscure cysts in the pineal gland were ignored; only the pineal cysts > 5 mm were examined. The cysts were encountered in 79 (1.3%) of 6023 patients evaluated. The cysts predominantly occurred in females; we found 29 cysts in 3008 males and 50 cysts in 3015 females (P < 0.05). Young adults had higher incidence of these cysts, with a peak incidence occurring between the ages of 21 and 30 years, then the incidence gradually decreased with age. We found no significant age distribution in males. However, among all groups, young women between the ages of 21 and 30 years had the highest frequency (5.82%). The incidence of pineal cysts among women between the ages of 21 and 30 years was significantly higher than in any other group (P < 0.001; 21-30 versus 51-60, 61-70, and 71-80). In females, when the incidence below the age of 41 was compared with that above or equal to 41, the z score was 4.98 (P < 0.001). The cystic expansion of the pineal gland in females appeared to begin at adolescence, and presumably the pineal gland decreases in size with aging. In this article, we will also speculate about the relationship of these changes to the secretory activity of the pineal glands.