The aim of the study was to evaluate risk factors for invasive cervix cancer in young British women and to look for factors which might explain its increased incidence in younger ages. A case-control study involving 121 women with invasive cervix cancer diagnosed before the age of 40 years, and 241 matched controls was undertaken. Questions were asked about demography, sexual behaviour, reproductive and contraceptive history, smoking, diet and hygiene. Age at first intercourse and lifetime number of sexual partners were found to be the most important factors, whereas parity, educational attainment and social class were not related to risk. Smoking was only weakly related to risk (P = 0.01) and this effect disappeared after adjustment for sexual behaviour variables. A weak protective effective of increased fruit consumption was observed (P = 0.03), but again this became non-significant after adjustment for sexual behaviour. Only factors related to sexual behaviour were found to be important in this study and other cofactors often associated with invasive cancer in older women were not found to be involved. It is possible that genetic factors influencing viral persistence, and specific more virulent strains of human papillomavirus may be more important in early onset cases.