An everting applicator for brachytherapy of body cavities such as the vagina has a flexible sleeve secured to a distal mandrel, both the sleeve and the mandrel having internal lumens. The flexible sleeve has a diverging opening at its distal end, preferably bell-shaped or cone-shaped, such that when the open end is pushed against the mouth of a vagina, the sleeve will evert back upon itself, progressively unrolling to an inside out configuration wherein, fully inserted, the sleeve is fully everted back over the exterior surface of the mandrel. A radiation source, isotopic or electronic, is then inserted into the mandrel lumen to commence a therapeutic irradiation procedure of tissues of the vagina.