Recent use of oral contraceptive pills is associated with a modest risk of breast cancer among very young women. In this US population-based case-control study, we evaluated whether the excess risk associated with recent oral contraceptive use is ubiquitous for all pill types or attributable to specific oral contraceptive preparations. Hormonal content and potency of combination oral contraceptives used for the longest duration within 5 years of interview for breast cancer cases aged 20-44 years (N=1640) were compared with age-matched community controls (N=1492). Women who recently used oral contraceptives containing more than 35 microg of ethinyl oestradiol per pill were at higher risk of breast cancer than users of lower dose preparations when compared to never users (respective relative risks of 1.99 and 1.27, P(trend)<0.01). This relationship was more marked among women <35 years of age, where risks associated with high- and low-dose ethinyl oestradiol use were 3.62 and 1.91 (P(trend)<0.01), respectively. We also found significant trends of increasing breast cancer risk for pills with higher progestin and oestrogen potencies (P(trend)<0.05), which were most pronounced among women aged <35 years of age (P(trend)<0.01). Risk was similar across recently used progestin types. Our findings suggest that newer low-potency/low oestrogen dose oral contraceptives may impart a lower risk of breast cancer than that associated with earlier high-potency/high-dose preparations.