Serum specimens from 75 women with cervical carcinoma in situ, 84 with squamous dysplasia, and 132 controls, who had previously been interviewed and tested for complement fixing antibodies against a number of organisms, were analyzed for HSV-2 antibodies. Carcinoma in situ and severe dysplasia were associated with HSV-2 antibodies. Mild dysplasia was related to evidence of prior infection by Trichomonas vaginalis, adenoviruses, and Mycoplasma pneumoniae, plus a history of vaginal discharge. Severe dysplasia was less strongly related to these variables. The relative risk of dysplasia increased with the number of different pathogens by which a woman had been infected. It is concluded that HSV-2 may be a cause of carcinoma in situ; that much dysplasia is a nonspecific reaction of the cervical epithelium to chronic inflammation; and that dysplastic lesions that are caused by HSV-2, and hence may be a precursor to carcinoma in situ, tend to be distinguished by their severity.