The objective of this study was to determine the effect of dietary phytoestrogens on the incidence of spontaneous vulvar carcinomas in 129/J mice using three natural ingredient diets and two purified diets containing predetermined levels of daidzein and genistein. Eighty weanling female mice without clinical evidence of vulvar carcinomas were randomly assigned 16 per diet to each of 5 test diets. Mice were clinically examined for vulvar masses weekly for 3 months and at monthly intervals thereafter. Vulvar carcinomas in representative groups of mice were confirmed using routine histological procedures. The incidence of vulvar carcinomas increased sharply in mice on all test diets during the first 2 months with minor changes during the remainder of the study. Within one month, the incidence of vulvar carcinomas in mice fed the AIN-76A modified soy protein diet was significantly (P < .05) increased over those of mice fed the AIN-76A modified casein diet, the #5K96, or the # 5058 diet. At three months, the incidence of vulvar carcinomas in mice fed the soy protein diet was significantly (P < .05) increased over those of mice fed the NIH-31 diet or the PMI #5K96 diet. There was a marginally significant (P < .10) correlation between the total daidzein and genistein levels in the five test diets and the incidence of vulvar carcinomas in mice as determined by clinical examination. We concluded that dietary levels of daidzein and genistein were associated with an increase in the incidence of vulvar carcinomas in mice and that the 129/J mouse may provide an animal model for studying the development of vulvar carcinomas.