Tailored telephone counseling (TTC) is effective in increasing utilization of screening mammography, but has received limited testing on a large scale in a contemporary HMO setting in which most eligible women get regular screening. We conducted a randomized controlled trial comparing TTC to an active control (mailed reminders) among women aged 50-80 enrolled in two HMOs in New England (n=12,905). Over a 1-year period counselors attempted to contact women in the intervention arm who had not had a mammogram within the last 15 months. The absolute increase in mammography use due to the intervention was 4.9% (OR 1.3, 95% CI 1.0-1.6) in one HMO and 3.1% (OR 1.2, 95% CI 1.0-1.3) in the other. We estimated that one additional woman was screened for each 10.9 women eligible for counseling. An intervention process analysis documented a high level of acceptance of TTC and identified subgroups that could be targeted for counseling to improve the efficiency of TTC.