Examples of cross-, concomitant- and pseudo-cross allergies are discussed based on the results obtained in a series of patch tests made with 31 chemical allergens. Special attention is paid to the relevance of positivity. Para-group allergy is analysed as an example of cross-allergy. Analysis of various combinations of allergy to p-phenylene diamine and phenylisopropyl-p-phenylene diamine suggests that in addition to the classical cross-allergy, concomitant sensitization may also occur. Concomitant allergy to three metals (chromium, cobalt and nickel) is discussed. Concomitant sensitization to chromium and cobalt has been found to occur most frequently. Associated positivity to various aromatic balsam-like substances (balsam of Peru, turpentine, colophony, wood tar) has been interpreted as pseudo-cross-allergy.