The most common mutation of the cystic fibrosis gene is characterized by the deletion of three nucleotides that code phenylalanine in the 508 position of the cystic fibrosis transmembrane conductance regulator. We report the first measurements by matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry for the delta F508 mutation in cystic fibrosis carriers and patients. Furthermore, in a blind test, results from the normal and delta F508 mutant alleles in 30 clinical samples based on MALDI mass spectrometry and on conventional gel analysis of the DNA were in total agreement. These results demonstrate the utility of MALDI mass spectrometry in the molecular diagnosis of mutant alleles and point to its potential use for ultra-fast detection in large-scale screening of DNA mutations.