Aim of the study was to verify the reliability in clinical practice of patch testing with "standard" series and additional series of haptens for the diagnosis of occupational and non-occupational allergic contact dermatitis, evaluating positive reactions and relating those reactions to professional categories. A total of 392 out of 937 patients (41.8%) showed at least one positive reaction to "standard" series testing; the hapten most frequently noted as the cause of positive reaction was nickel sulphate. Professional categories that showed positive reactions to "standard" series most frequently were clerks, hairdressers and hospital auxiliary workers. Among 897 patients tested with nonstandard allergens, only 124 (13.8%) elicited at least one positive reaction, ammonium persulphate being the most frequently positive hapten. A dominant percentage of positive results was seen in hairdressers and cleaning personnel. No positive reactions were observed in a large number of haptens, tested more than 200 times. Haptens of "standard series" elicited a higher number of positive reaction than the additional series, even though there was a high specificity of few additional series haptens in some professional categories. Data suggest some caution in systematically testing additional series, despite a higher accuracy and diagnostic efficacy in some job categories.