OBJECTIVES: To evaluate the safety and efficacy of botulinum toxin type A in the treatment of bilateral primary axillary hyperhidrosis. DESIGN: Multicentre, randomised, parallel group, placebo controlled trial. SETTING: 17 dermatology and neurology clinics in Belgium, Germany, Switzerland, and the United Kingdom. PARTICIPANTS: Patients aged 18-75 years with bilateral primary axillary hyperhidrosis sufficient to interfere with daily living. 465 were screened, 320 randomised, and 307 completed the study. INTERVENTIONS: Patients received either botulinum toxin type A (Botox) 50 U per axilla or placebo by 10-15 intradermal injections evenly distributed within the hyperhydrotic area of each axilla, defined by Minor's iodine starch test. MAIN OUTCOME MEASURES: Percentage of responders (patients with >/=50% reduction from baseline of spontaneous axillary sweat production) at four weeks, patients' global assessment of treatment satisfaction score, and adverse events. RESULTS: At four weeks, 94% (227) of the botulinum toxin type A group had responded compared with 36% (28) of the placebo group. By week 16, response rates were 82% (198) and 21% (16), respectively. The results for all other measures of efficacy were significantly better in the botulinum toxin group than the placebo group. Significantly higher patient satisfaction was reported in the botulinum toxin type A group than the placebo group (3.3 v 0.8, P<0.001 at 4 weeks). Adverse events were reported by only 27 patients (11%) in the botulinum toxin group and four (5%) in the placebo group (P>0.05). CONCLUSION: Botulinum toxin type A is a safe and effective treatment for primary axillary hyperhidrosis and produces high levels of patient satisfaction.