It has been suggested that increased numbers of ovulations might increase the risk of p53 gene (also known as TP53) mutation in the ovarian epithelium, thereby leading to the development of cancer. The data supporting this hypothesis have come from an observation that accumulation of p53 protein in epithelial ovarian cancer was strongly associated with increasing numbers of ovulatory cycles. We have further investigated the association between ovulatory history and p53 gene mutation by use of data from a large case-control study of ovarian cancer in Australia. Tissue blocks were available for immunohistochemical analysis of p53 protein from 234 case subjects, aged 18-79 years, who had invasive epithelial ovarian cancer. Epidemiologic data were also available for these women and for 855 control subjects. Case-case comparisons were made by use of prevalence ratios and 95% confidence intervals (CIs), and case-control comparisons were made by use of odds ratios (ORs) and 95% CIs. All statistical tests were two-sided. There was no association between p53 accumulation and years of ovulation. Women with p53-positive cancers had undergone an average of 29.3 years of ovulation compared with 29.0 years of ovulation for women with p53-negative cancers (P=.8). Although the overall risk of ovarian cancer development was significantly increased in women who had undergone more years of ovulation (OR=2.17; 95% CI =1.54-3.05-for > or =35 years versus <23 years of ovulation), there was no difference in the risk associated with p53-positive and p53-negative cancers. These results confirm the association between increased ovulation and ovarian cancer risk but do not support the hypothesis that this association is due to an increased risk of p53 mutation with a greater number of ovulatory cycles.