LHRH analogue therapy has proved effective in the treatment of endometriosis. Although in the majority of patients a marked reduction in AFS implant score is observed, in some cases residual endometriotic foci are macroscopically suspected and can be confirmed by histological examination during a second check operation at the end of treatment. In the study presented, 34 patients from 69 showed residual endometriosis after a 6 months' treatment with the LHRH analogue buserelin (900 mcg/d intranasally). According to light microscopical criteria regressive changes were seen in 52% of the implanted patients, nevertheless 29% presented with proliferative elements of the disease. In 19% proliferation and regression were seen in the same biopsy or even in the same gland. Histology did not relate to either the degree of ovarian suppression by LHRH analogue therapy, or to the location of the residual endometriotic foci. These results add another piece of evidence to the fact, that endometriosis presents itself as a heterogenic disorder. In addition, susceptibility to a hormonal ablative therapy rather depends on the degree of dysontogenetic differentiation, than on the completeness of induced ovarian suppression.