Atypical hyperplasia (AH) is a lesion known to be associated with a marked elevation of risk for subsequent development of breast cancer. Like ductal carcinoma in situ (DCIS), it is more commonly found by mammography. We studied all cases designated as AH, DCIS and ductal carcinoma in situ with microinvasion (DCISM) at the University of Alabama at Birmingham USA from 1984-1988. Clinical records, mammograms, and pathologic material were reviewed on 22 cases of AH (21 patients), 41 cases of DCIS (40 patients) and 14 cases of DCISM. Women with AH had a lower average age than those with either DCIS or DCISM. A family history of breast cancer in a first degree relative was present in 38% of women with AH, 25% of women with DCISM. These pathologic lesions were found in association with mammographically dense tissue in 50% or more of cases. Distinctive calcifications were present in 24% of DCIS lesions and 70% of patients with DCISM. While mammographic microcalcification was the indication for biopsy in 59% of cases of AH, the calcifications were most often in benign proliferative disease elsewhere in the specimen rather than in the atypical focus itself. In 82% of AH, the lesion extent was less than 5 mm in size, while 83% of DCIS lesions and 93% of DCISM lesions were larger than 5 mm. Atypical hyperplasia, unlike many cases of DCIS and DCISM, has no direct mammographic correlate in contrast to DCIS and DCISM, which often have distinctive features on mammography. Careful correlation of the relative size, extent, and morphology of these lesions is essential to appropriate management and counselling of these patients. © 1993.