BACKGROUND Focal nodular hyperplasia (FNH) of the liver is a rare benign nodular lesion that arises in women of reproductive age. Although a role of female hormones has been suggested, their influence on the course of FNH has remained controversial. CASE REPORT A 44-year-old woman with a 12-year history of oral contraceptive use was referred to our hospital for examination of an asymptomatic liver mass (3 cm in diameter) identified by computed tomography. We diagnosed FNH using imaging methods and fine-needle biopsy. Oral contraceptives were discontinued because the mass increased over a period of 21 months. Four months later, the mass had decreased in size, indicating that FNH can spontaneously regress when oral contraceptives are discontinued. CONCLUSIONS Discontinuation of oral contraceptives use can reduce the size of FNH, as in this case.