Cervical biopsy specimens from 422 women attending a colposcopy clinic showed various grades of dysplasia in 387 and no evidence of dysplasia in 35. Of the women with dysplasia (CIN I, II or III) 67% were smokers and 33% were non-smokers compared with 43% smokers and 57% non-smokers in those without evidence of dysplasia. Of the women with CIN I 56% were smokers and 44% were non-smokers; 66% of those with CIN II were smokers and 34% were non-smokers (P less than 0.02), and 71% of those with CIN III were smokers (P less than 0.01). There were no significant differences between the smokers and non-smokers in the proportion of women who had had either a miscarriage or an abortion, in the prevalence of condylomata acuminata, and the use of oral contraception or barrier methods. Neither were there any significant differences in numbers of patients seropositive for cytomegalovirus or in CMV antibody titres among the groups. The detection of koilocytic cells in cervical biopsies showed a significant difference between smoking and non-smoking groups which reflected a significant increase in koilocytosis in smokers with CIN III.