Forty-six patients with biopsy-proven cervical koilocytosis, which in 19 cases was associated with CIN I, were randomized to treatment by cervical laser vaporization or to follow-up without therapy. The polymerase chain reaction was used to examine the paraffin-embedded cervical biopsies for human papillomavirus (HPV) types 6, 11, 16, 18 and 33 by polymerase chain reaction and molecular hybridization technique. Twenty biopsies contained HPV DNA, in 17 cases of the putatively oncogenic types HPV 16 and HPV 18. During a mean follow-up of 28 months, the lesions resolved in 37 women, 5 women had progression to CIN III lesions, and 4 women had persisting disease. Lesions not harboring any of the 5 HPV types showed a high spontaneous regression rate of 88%, which was not improved by the laser therapy. In the HPV-positive lesions, resolution occurred in only 29% of untreated lesions, but in 92% of those treated by laser. The difference is significant (P = 0.007). It is concluded that in patients with cervical koilocytosis, the HPV diagnosis could be of practical value in identifying patients who might benefit from treatment.