BACKGROUND: Fluctuating irritability of the skin induced by low ambient temperature and humidity may compromise the reproducibility of patch testing. OBJECTIVES: To assess the impact of temperature and absolute humidity at the time of patch testing on the occurrence of irritant or doubtful (IR/?), weak positive (+) and (strong) positive (++/+++) reactions, respectively, among 12 allergens included in the German Standard Series. METHODS: Analysis of clinical data collected in the surveillance network IVDK (http://www.ivdk.org) between January 1993 and December 2001 (n Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) 73 691 patients) combined with meteorological data obtained by the national services in Germany and Austria. Multinomial logistic regression analysis was used to estimate the risk associated with temperature, absolute humidity and vapour pressure, respectively, adjusted for sex, age, atopic dermatitis and duration of patch test application. RESULTS: For low temperature and humidity, a relevant increase of IR/? reaction frequency was observed in the cases of paraben mix and (chloro-) methylisothiazolinone. Both IR/? and + reactions were significantly increased with respect to the allergens fragrance mix, oil of turpentine, methyldibromo glutaronitrile + phenoxyethanol and particularly formaldehyde, while ++/+++ reactions were hardly affected by weather conditions. CONCLUSIONS: The observed increase of IR/? reactions may be due to epidermal barrier function impairment. The impact of dry/cold weather on + reactions in terms of possibly false-positive reactions is restricted to few allergens. In the case of + reactions of unknown relevance, a re-test under warm conditions or verification tests such as the repeated open application test or the provocative use test may be recommendable.