Background: Eccrine hydrocystomas are essentially eccrine sweat gland retention cyst located in the dermis that clinically presents as small translucent cystic swelling. Objective: This study was designed to evaluate the efficacy and safety of 20% aluminum chloride hexahydrate in absolute alcohol in the treatment of eccrine hydrocystoma. Patients and Methods: In a randomized, double-blind, placebo controlled clinical trial, 33 patients with multiple eccrine hydrocystoma referred to Tehran Razi hospital during the summer of 2000, were divided in two groups: One group received 20% aluminum chloride hexahydrate in absolute alcohol and the other one received only absolute alcohol. The patients applied the drugs to dried skin at bedtime, and washed the area in the morning. Results: Excellent or good therapeutic effects (More than 50% of improvement) were achieved in 87.5% of patients who applied 20% aluminum chloride hexahydrate but only in 17.6% of patients in control group (P<0.0001). The mean age in the first group was 42.6 years and in placebo group 45.4 years. Side effects in the first group included pruritus, burning (37.5%), xerosis and scaly (31.2%) and erythema (25%), in control group the rate of these side effects were 23.5%, 17.6%, and 11.7% consequently. Conclusion: 20% aluminum chloride hexahydrate solution is an effective and safe topical agent in the treatment of multiple eccrine hydrocystomas.