A study population of 1,158 paid adult volunteers was obtained. Prior to patch testing, a history of previous exposure to four allergens also was obtained. Prevalence of positive reactions to patch tests was nickel, 5.8%; neomycin, 1.1%; ethylenediamine, 0.43%; and benzocaine, 0.17%. Nine percent of women reacted to nickel compared with 0.9% of men. There was a strong correlation of nickel sensitivity with a history of pierced ears, earlobe rash, and jewelry rash. Ten of 12 neomycin-positive subjects used neomycin for one week or longer on an inflammatory dermatosis, compared with six of 36 age-, race-, and sex-matched controls. By history, 85% were exposed to benzocaine, 48% to neomycin, and 15% to Mycolog (ethylenediamine). Of 127 patients referred to clinics for evaluation of contact dermatitis, 11% yielded positive tests to nickel, 6.3% to neomycin, 3.1% to ethylenediamine, and 1.6% to benzocaine. Data obtained from testing contact dermatitis patients are not applicable to the general population.