Incidence rates of ductal carcinoma in situ (DCIS) and lobular carcinoma in situ (LCIS) have increased rapidly over the past several decades largely due to the increased use of mammography. However, recent data from 1987 to 1999 indicate that invasive ductal carcinoma incidence rates have remained essentially constant, whereas rates of invasive lobular carcinoma have increased 65%, with greater increases observed among postmenopausal women. Data on recent trends in DCIS and LCIS incidence rates, particularly age-specific trends, are lacking. We evaluated trends in the incidence rates of DCIS overall, noncomedo DCIS, comedo DCIS, and LCIS using data from nine Surveillance, Epidemiology, and End Results cancer registries. DCIS incidence rates increased 7.2-fold [95% confidence interval (95% CI), 6.8-7.7] from 1980 to 2001, 1.8-fold (95% CI, 1.7-1.9) over the past 10 years (1992-2001), and 1.1-fold (95% CI, 1.0-1.2) over the past 5 years (1997-2001). The magnitudes of these increases were highest among women ages > or = 50 years. Furthermore, over the past 10- and 5-year periods, rates of noncomedo DCIS have generally increased across all age groups, whereas rates of comedo DCIS held constant or decreased. LCIS incidence rates increased 2.6-fold (95% CI, 2.3-2.9) from 1980 to 2001, 1.3-fold (95% CI, 1.2-1.5) over the past 10 years, and 1.1-fold (95% CI, 1.0-1.3) over the past 5 years. Similar to invasive lobular carcinoma, but unlike invasive ductal carcinoma, incidence rates of both DCIS and LCIS continue to increase in the United States primarily among older women. These trends present important public health and clinical challenges.