Magnetic resonance imaging (MRI) allows the diagnosis of adenomyosis in vivo with a high sensitivity and specificity. Usually the diagnosis of adenomyosis is obtained from women in their fourth to fifth decade of life. However, recent data suggest that adenomyosis may develop much sooner in life, particularly in women with endometriosis. In order to test these suggestions, MRI of the uterus in 227 women with and without endometriosis was performed and the results were related to the age of the subjects (age groups: 17-24, 25-29, 30-34 and >34 years). The study revealed that the process of the development of adenomyosis, represented by an increased diameter of the dorsal junctional zone of the uterus as the imaging correlative of the invasion of basal endometrium into the junctional zone, had already commenced early in the third decade of life and progressed steadily during the fourth decade in women with endometriosis. Women without endometriosis showed almost no signs of adenomyosis up to the age of 34 years. Surprisingly, parallel in both groups of women, a marked increase in the incidence of adenomyosis could be observed beyond the age of 34 years, thus representing a common phenomenon in the age-related pathophysiological continuum of adenomyosis.