The value of cervical (Papanicolaou) smears in monitoring the natural history of cervical human papillomavirus (HPV) infections was assessed in a series of 513 women prospectively followed since 1981. On each clinic visit, the patients were subjected to colposcopy accompanied by cervical smears and/or punch biopsies. The latter were analyzed by light microscopy for concomitant cervical intraepithelial neoplasia (CIN) and by transmission electron microscopy (TEM) for HPV particles as well as for HPV structural proteins. The stromal immunocompetent cell (ICC) infiltrates were phenotypically characterized using monoclonal antibodies for T-cell subsets, NK and K cells and Langerhans cells. HPV DNA typing was accomplished by Southern blot, spot and in situ hybridization using probes for HPV 6, 11, 16, 18 and 31. Lesions showing only changes consistent with HPV infection (HPV-NCIN) were associated with less severe atypia in cervical smears than were lesions with coexistent CIN (HPV-CIN). Normal smears were observed, however, in 24.7% of the cases with HPV-NCIN lesions, in 11.5% of cases with HPV-CIN I lesions but only exceptionally in cases with HPV-CIN II and III lesions (2.2% and 3.3%). The percentages of the different ICC phenotypes did not correlate with the atypia in cervical smears, but there was a shift towards the lower values of the T-helper/T-suppressor (OKT4+/OKT8+) cell ratio in parallel with increasing atypia. The possibility of latent HPV infection was suggested by the detection of viral particles, HPV antigens and HPV DNA in lesions shedding normal cells in the smears. The high-risk HPV types 16 and 18 were associated with the highest frequency of severely atypical cells; in the majority of cases, the low-risk types HPV 6 and 11 presented with less severe atypia. The first cervical smear seems to be of value as a predictor of the natural history of HPV lesions, as indicated by the fact that regression was inversely and progression directly related to initial cellular atypia. The present results confirm the intimate association between HPV infections and CIN. Although the biologic potential of the HPV infections seems to be dependent on multiple factors, routine cervical smears, because of their potential value in monitoring the natural history of this infection, should constitute an important means in the prospective follow-up of these patients.