In September 1982, two Charlottesville, Virginia, residents were found by their dentists to have general erosion of dental enamel consistent with exposure to acid. Both patients were competitive swimmers at the same private club pool. No other common exposure could be determined. An epidemiologic survey was made of 747 club members. Symptoms compatible with dental enamel erosion were reported by 3% of nonswimmers (9/295), 12% of swimmers who were not members of the swim team (46/393), and 39% of swim team members (23/59). All four swimmers with clinically verified dental enamel erosion had trained regularly in the pool for competitive swimming meets, compared with one of eight matched swimmers without enamel erosion. Examination of the implicated swimming pool revealed a gas-chlorinated pool with corrosion of metal fixtures and etching of cement exposed to the pool water. A pool water sample had a pH of 2.7, i.e., an acid concentration approximately 100,000 times that recommended for swimming pools (pH 7.2-8.0). A review of pool management practices revealed inadequate monitoring of pool water pH. Acid erosion of dental enamel--"swimmer's erosion"--is a painful, costly, irreversible condition which can be caused by inadequately maintained gas-chlorinated swimming pools.