Purpose: To determine the effectiveness of a community-based intervention to increase the use of screening mammography among disadvantaged women at an inner-city drop-in center. Methods: This study involved women 50 to 70 years old who were clients of an inner-city drop-in center in Toronto, Canada, during the years 1995-2002 (N = 158 in 1995-2001 and N = 89 in 2002). In 2002, the drop-in center and a nearby hospital initiated a collaborative breast cancer screening project in which a staff member of the drop-in center accompanied small groups of women for mammography visits at a weekly pre-arranged time. Interrupted time series analysis was used to examine the effect of this intervention on the annual rate of screening mammography, as determined by review of medical records. Results: More than half of the women 50 to 70 years old who used the drop-in center in 2002 had been diagnosed with a major mental illness, and one-third were either homeless or living in supportive housing. In the 7 years before the introduction of the intervention, annual mammography rates among women using the drop-in center averaged 4.7%. During the intervention year, 26 (29.2%) of 89 women underwent mammography (p = 0.0001 for the change from pre-to post-intervention). Conclusions: The introduction of accompanied small-group visits was associated with significantly increased use of mammography in a group of disadvantaged women who were clients of an inner-city drop-in center. This approach may be useful to promote breast cancer screening among women affected by mental illness or homelessness who have contact with community-based agencies.