We analyzed DNA of ten cystic fibrosis (CF) patients representing DNA of 19 different CF chromosomes and screened for deletions by means of field inversion gel electrophoresis (FIGE). No differences were detected after digestion of the DNA samples with the restriction enzymes Not I and Sfi I and hybridization with the probes MetH, MetD, J3.11, and 7C22. Thus the percentage of deletions occurring within the CF region and detectable with FIGE is less than 15.2% (95% confidence interval, N = 19).