We aimed to retrospectively analyze the risk factors of a continuous dienogest (DNG) therapy for serious unpredictable bleeding in patients with symptomatic adenomyosis. This is a retrospective study based on data extracted from medical records of 84 women treated with 2 mg of DNG orally each day between 2008 and 2017. 47 subjects were excluded from the original analyses due to an inadequate subcategorization into subtype I and subtype II and a lack of hemoglobin levels. The influence of various independent variables on serious unpredictable bleeding was assessed. Of the 37 eligible patients who received the continuous DNG therapy, 14 patients experienced serious unpredictable bleeding. Univariate analysis revealed that the serious bleeding group had subtype I adenomyosis (P = 0.027). There was no correlation between age, parity, minimum hemoglobin level before treatment, previous endometrial curettage, and duration of DNG administration, or uterine or adenomyosis size and the serious bleeding. A DNG-related serious unpredictable bleeding is associated with the structural type of adenomyosis (subtype I) in patients with symptomatic adenomyosis.