An attempt was made to evaluate the relationship between histologic findings of adenomyosis and onset of dysmenorrhea. Thirty-four cases of histologically diagnosed adenomyosis were investigated in this study. One large sample cut along the longitudinal axis was made and examined to evaluate the histologic parameters and CA 125 levels. Dysmenorrhea was classified into grade I (absent or mild) and grade II (severe). Localization of adenomyosis was divided into type I, adenomyosis continuous from the surface endometrium (29 cases), and type II, adenomyosis present on the serous side of the uterus and not continuous from the surface endometrium (five cases). We made type I adenomyosis, which included 13 cases of grade I and 16 cases of grade II, the focus of our investigation. The parameters that correlated with dysmenorrhea were the number of islands of adenomyosis and glands, distance of invasion, relative rate of invasion, and CA 125 levels. The differences between grades I and II were apparent in the frequency distribution tables, with dysmenorrhea occurring at invasion rates of 80% and greater.