Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene cause the hereditary disease cystic fibrosis. The most frequent mutant deltaF508 has been shown in vitro to be retained in the endoplasmic reticulum. Ex vivo studies using immunohistochemical labelling in cryofixed skin biopsies have confirmed the mislocalization of deltaF508 CFTR in sweat glands. The purpose of this study was to test CFTR antibodies in paraffin-embedded skin biopsies to take advantage of the superior tissue preservation as compared to cryofixation. A panel of 7 CFTR antibodies was applied to skin sections of healthy controls and of cystic fibrosis patients homozygous for the deltaF508 mutation. Sweat gland labelling consistent with CFTR localization and different between control and cystic fibrosis tissue was obtained with 2 antibodies. Conventional staining controls confirmed the labelling specificity. The antibodies were subsequently tested in a series of 237 sections of 16 biopsy specimens. However, the sweat gland labelling pattern proved not to be dependent on CFTR genotype. This finding was the sole indicator of non-specificity of the staining which was revealed only by the size of our random sample. Our results emphasize that CFTR immunolabelling following formalin fixation has to be interpreted with the utmost caution.