A 19-year-old woman, who had been receiving hormone replacement therapy for 13 months before the diagnosis of mosaic Turner syndrome (46XXp-/45X), developed Crohn's colitis and erythema nodosum of the lower legs. Colonoscopy revealed an anal fistula and the presence of deep longitudinal ulcers with cobblestoning in the colorectum. Therapy with prednisolone and adalimumab was effective for the intestinal and skin lesions. To date, all seven case reports of Turner syndrome in Japan have also developed Crohn's disease after hormone therapy, suggesting a possible association of sex hormones in the pathogenesis.