Little is known about the influence of progestogen-only oral contraceptives on a woman's risk of breast cancer. This issue was examined in a national population-based case-control study in New Zealand. A total of 891 women aged 25 to 54 years with a first diagnosis of breast cancer and 1,864 control subjects, randomly selected from the electoral rolls, were interviewed. Use of progestogen-only pills was reported by 8.7 percent of all control subjects (and by 17.3 percent of those aged 25 to 34 years). The relative risk (RR) of breast cancer in women who had ever used progestogen-only pills was estimated to be 1.1 (95 percent confidence interval [CI] = 0.73-1.5). In women aged 25 to 34 years, the RR was 2.3 (CI = 1.2-4.3). Women who had started using progestogen-only pills within the last 10 years were at increased risk of breast cancer (RR = 1.6, CI = 1.0-2.4), whereas those who had first used them earlier were at significantly reduced risk (RR = 0.44, CI = 0.22-0.90). These findings are similar to results for depot medroxyprogesterone acetate, and a possible analogy with the influence of pregnancy is also suggested.