Women are born with a non-renewable pool of ovarian follicles, referred to as the ovarian reserve. This reserve consists of primordial follicles in the ovaries and can be affected by many factors, such as genetic and endocrine disorders, medical interventions, and endocrine disruptors. Fertility preservation is recommended when gonadotoxic treatments are necessary. The preferred options for women are oocyte and embryo cryopreservation. However, in very young, sexually immature patients, ovarian tissue cryopreservation is the only option. Knowing the follicular density of cryopreserved tissue samples is essential in fertility counseling for young patients. This protocol demonstrates the use of Z-scores for cortical follicle density as a tool to evaluate the quality of ovarian tissue in girls and young women aged 25 years and below who are undergoing fertility preservation. Follicle density in patient samples is compared to age-normalized reference standards, developed by Hassan et al. to estimate possible deviations from the standard. The follicle density is measured through histological quantification. For this, a small piece of ovarian cortical tissue (~2 mm x 2 mm x 2 mm) is fixed in either Bouin's or formaldehyde solution, embedded in paraffin, sectioned at 4 µm thickness, stained with hematoxylin and eosin, and digitized using a slide scanner. Follicular stages present in the cortex range from primordial to primary follicles. The cortical area was 1 mm from the surface epithelium on histological sections. Follicle density is calculated using a modified formula presented by Schmidt et al., and the Z-score is determined using the reference mean and standard deviation. The Z-score indicates how much the measured value deviates from the reference mean, determining reduced (<-1.7 Z-score) ovarian reserve. With this method, follicle densities can be used as a valuable measure of ovarian reserve for young patients requiring fertility preservation.