A series of eleven comparative wart treatment trials undertaken between 1969 and 1975 and involving 1802 patients is described. A method of coding provided groups of patients matched for age, type, number and duration of warts, within which treatments could be randomized. The variation in response to treatment was shown to be influenced significantly by these factors and the level of cure to depend on the proportions of the various response groups within the population under consideration. These proportions were found to vary geographically and at different periods. In all the trials the results were assessed at 12 weeks. In the treatment of hand warts, the percentage cure of patients treated with liquid nitrogen fell significantly from 75 to 40% when the interval between freezings was increased from 3 to 4 weeks. The average number of freezings required for a cure was 3-1 amongst all patients cured by 6 or less freezings at intervals of 2 or 3 weeks. In a two-centre trial there was no significant difference between the percentage cure of patients with hand warts treated with liquid nitrogen (69%) and of those applying a paint containing salicylic and lactic acids (SAL) (67%). Patients receiving both treatments concurrently did better (78%) but the difference was not found to be statistically significant. In the treatment of simple plantar warts the percentage cure for the SAL paint (84%) was found to compare favourably with that for a podophyllin treatment (81%). Only one of the patients cured by the paint in that trial was found to have had a recurrence after 6 months. The paint was found to be satisfactory for use under general practice conditions. Additions to the formula did not alter its effectiveness. In the treatment of mosaic plantar warts the overall percentage cure for the SAL paint in a series of comparative trials (1969-75) was 45%. In these trials it was compared directly with one or more other preparations. No differences were found between its efficacy and that of 10% buffered gluteraldehyde (47%), 40% benzalkonium chloride dibromide (Callusolve 40) (30%) and 5% 5-fluorouracil in dimethyl sulphoxide (53%). Only 25% of thirty-six patients treated with 5% idoxuridine in dimethylsulphoxide were cured. Throughout the trials approximately 30% of patients with hand warts, 20% of those with simple plantar warts and 50% of those with mosaic plantar warts were found to be resistant to treatment. The adoption of treatment with SAL paint for hand warts and simple plantar warts by the general practitioners in the Edinburgh area has proved satisfactory. Only resistant cases are now referred to hospital and these can be treated within a few weeks instead of 4-5 months as was the case in 1969.