To analyse the prevalence of (pre) malignant lesions occurring in breast and adnexal tissue at prophylactic surgery in women at hereditary high risk of breast and/or ovarian cancers. Tissue was obtained from 85 women who underwent prophylactic bilateral salpingo-oophorectomy (pBSO) and from 59 women who underwent prophylactic mastectomy (pM). Control tissue samples were obtained from women undergoing breast reduction surgery (N = 99) or adnexal surgery for benign reasons (N = 72). In women with a BRCA1/2 mutation, the prevalence of a (pre) malignant adnexal lesion was 50% (95% CI 26-74) if older than 40 years and 14% (95% CI 0-58) if younger. The prevalences of (pre) malignant breast lesions in women older than 40 years, with and without a BRCA1/2 mutation, were 0% (95% CI 0-16) and 47% (95% CI 21-73), respectively. No association was found between (pre) malignant lesions in breast and adnexal tissue occurring in 28 women who underwent surgery on both organs (R = 0.155, p = 0.432), but the prevalence of lesions was significantly higher in adnexal tissue than in the breast (p = 0.023). Compared to controls, women at hereditary high risk had a higher chance of (pre) malignant lesions in the breast and an even higher chance of such lesions in the adnexal tissue. There was no indication for concomitant presence of such lesions in both organs at the time of prophylactic surgery. The high frequency of (pre) malignant lesions in the adnexal tissue stresses further the importance of pBSO from the age of 40 onwards in women at hereditary high risk.