This study was undertaken to assess the effectiveness of CO2 laser (vaporization), 5-FU topical application and Interferon (IFN alpha-2a) parenterally in the therapy of penile intraepithelial neoplasia (PIN). From March 1986 to September 1991, 1,372 men, sexual partners of women with condylomata accuminata or flat or cervical intraepithelial neoplasia (CIN), were submitted to peoscopy, of 1,019 men who presented with various lesions caused by human papillomavirus (HPV)-confirmed histologically-208 were treated for PIN. The best treatment modalities irrespective of grade of lesion were found to be the combination of 5-FU plus CO2 laser vaporisation plus IFN alpha-2a (high dose) (96.15%), the combination of 5-FU plus CO2 laser vaporization (87.09%) and the combination of CO2 laser vaporization plus IFN alpha-2a (high dose) (80%). It is concluded that IFN alpha-2a (low dose) can be used as first line treatment in combination with 5-FU in patients with PIN II and as an adjuvant treatment (high dose) in patients with recurrent PIN I and PIN III.