A randomised, double-blind, single-centre, vehicle-controlled clinical trial was conducted to assess the efficacy and tolerability of diclofenac-Na 0.1% gel in 172 subjects suffering from acute first-degree natural sunburn. Overall 172 subjects with skin phototypes II-IV were randomised in a ratio of 2:1 to receive two applications of either diclofenac-Na 0.1% Emulgel gel or its vehicle Emulgel gel, 6 and 10 hours after the end of sun exposure. Subjects were drawn from a target population of healthy volunteers and well outdoor sunbathers with normal tolerance to ultraviolet light and the sun. Previously untanned areas were exposed to carefully determined standardised doses of sun (2.8 individual minimal erythema doses) on 15% body surface area to induce first-degree sunburn. After administration of diclofenac-Na 0.1% gel, subjects reported a significant reduction in spontaneous pain intensity compared with those on vehicle. Pain relief was rapid with a reduction in erythema, which was apparent within the first few hours after the first application of the trial medication with a maximum effect observed up to 30 hours after sun exposure. A good', very good', or excellent' cooling effect was recorded by 85% of subjects after treatment. Reported treatment-emergent adverse effects were infrequent, generally mild and none were considered to be related to the trial medication. Only one severe treatment emergent adverse event (abdominal pain) was recorded in the active group, and another (burning sensation) with vehicle.