Purpose: We examined the external genitalia of boys to estimate the age-specific prevalence of foreskin development before adolescence. Materials and methods: A total of 189 boys aged 0 to 13 years were enrolled in this study. The boys were categorized into four groups according to their age (group 1-4). The foreskin condition was classified as type I (normal prepuce), type II (adhesion of prepuce), type III (partial phimosis) and type IV (phimosis). Other abnormalities of the genitalia were also recorded. All the examinations were performed by the same urologist. Results: The incidence of type I foreskin was 46.6% in group 1 (age 0-1 year), 50.6% in group 2 (2-5 years), 77.3% in group 3 (6-9 years) and 46.2% in group 4 (10-13 years). The incidence of type IV foreskin was 20.7% in group 1, 19.3% in group 2, 4.5% in group 3 and 53.8% in group 4. Of the genital abnormalities, cryptorchidism was the most common (n=96), followed by hydrocele (n=61). Conclusion: Physiologic phimosis showed a tendency to decrease with age up to 10 years. Most of the boys with phimosis did not require treatment.