We investigated the increased incidence of early-stage breast cancer with micrometastatic lymph node involvement. Breast cancer incidence trends from 1990 through 2002 in the US Surveillance, Epidemiology, and End Results Program catchment area were analyzed. Joinpoint regression was used to show the annual percentage change (APC) in breast cancer incidence trends. The overall incidence of breast cancer among women aged 50-64 years increased 1.8% (95% confidence interval [CI] = 1.4% to 2.2%) per annum from 1990 through 2002 but decreased in all other age groups. Stage IIA and stage IIB tumor incidence increased (APC for stage IIA from 1996 to 2002 = 61.9%, 95% CI = 51.1% to 73.4%, and APC for stage IIB from 1998 to 2002 = 53.7%, 95% CI = 20.6% to 96.0%). The incidence of micrometastatic lymph node involvement for stage IIA and stage IIB tumors increased during the 1990s, especially after 1997 (APC = 17.3% for both stages), more for estrogen receptor-positive than estrogen receptor-negative disease. Increased use of mammography screening partly explains the increased incidence of early-stage breast cancer. Increases in small tumors with micrometastatic lymph node involvement may be attributable to the increased use of the sentinel lymph node biopsy in community practice.