The Free Androgen Index (FAI) was initially proposed as a measure for assessing the circulating testosterone availability in female hirsutism. The extension of its use, by a number of investigators, to males has not been formally justified. An analysis of its derivation from the Law of Mass Action reveals an implied assumption that the binding capacity of sex hormone binding globulin should greatly exceed the concentration of its ligand testosterone. This does not hold in adult males for whom the use of FAI is therefore inappropriate. A comparison of FAI and free-testosterone (determined by centrifugal ultrafiltration) yielded a correlation coefficient (r) of 0.858 for 20 adult females but only 0.435 for 19 adult males.