Background: Pattern hair loss (PHL) can be classified into several patterns. Currently, the Hamilton-Norwood classification system for men and the Ludwig grade system for women are commonly used to describe patterns of hair loss. However, these pre-existing classifications have some limitations. Objective: To establish an acceptable, universal, and accurate standard of both male and female pattern hair loss and to report its use in determining the incidence of PHL. Methods: We developed a new classification system (BASP classification) and then applied this system to classify the types of PHL. The BASP classification was based on observed patterns of hair loss. The basic (BA) types represent the shape of the anterior hairline, and the specific types (SP) represent the density of hair on distinct areas (frontal and vertex). There are four basic types (L, M, C, and U) and two specific types (F and V). The final type is decided by the combination of the assigned basic and specific types. Between November 2004 and June 2005, 2213 Korean subjects, comprised of 1768 males and 445 females, were classified according to the BASP classification at 13 university dermatologic centers nationwide throughout South Korea, as a multicenter study of the Korean Hair Research Society. Results: For both sexes, the majority of patients enrolled in the study were in the third and fourth decade of life (65.1% of males and 56.68% of females). In males, the older group as well as the younger group in the study were more likely to have little recession of the frontal hairline (classified as type M1∼2) and diffuse thinning over the top of scalp (type F1∼2). The women in the study developed typical female PHL. Limitations: The subjects of our study were mostly outpatients and some inpatients who complained about hair loss, not the general population of Korea. Conclusion: The BASP classification is a new stepwise, systematic, and universal classification system for PHL, regardless of sex.