A clinical trial was undertaken to compare the efficacy of neutral red photodynamic inactivation treatment of genital herpes infections with that of a non-photoactive dye, phenol red, as a control. In a series of nineteen patients with virologically proven herpes genitalis who were adequately followed, eleven were treated with neutral red, and eight with phenol red; no difference in response to therapy was found between the two groups, and it is concluded that under the conditions of this trial neutral red with photoinactivation was not effective in the treatment of acute genital herpes infections. Approximately 75 per cent. of those with vulval lesions also had concurrent cervical infection, so that an effective topical treatment would need to be applicable to both anatomical sites.