Twenty patients with post-herpetic neuralgia (median duration 28.5 months) were randomly allocated to receive transdermal iontophoresis of either vincristine or saline. Although significant improvement in pain by word score and visual analogue scale (P = 0.05) was reported by 6 out of 10 of the vincristine group, none of the patients considered themselves 'cured.' There was no significant change in the saline group. No adverse haematological or neurological side effects were seen, but skin irritation and painless electrical burns were common in both groups. The dramatic relief of pain in patients with post-herpetic neuralgia of 3 months or less reported elsewhere was not seen in our group who had pain of a longer duration. This present trial does not confirm the value of vincristine iontophoresis in the treatment of post-herpetic neuralgia of over 6 months duration.