In this paper the fitness of the delta F508 heterozygote is assessed and the age of the delta F508 mutation in the cystic fibrosis locus is estimated. Data from three microsatellite loci are applied. The analysis is performed conditional on the present-day frequency of the delta F508 mutation and based on assumptions about the demographic history of the European population and the mutation rate in the three microsatellite loci. It is shown that the data gives evidence of positive selection (up to 2-3% per delta F508 heterozygote), but also that data could be explained by negative selection of roughly the same order of magnitude. The age of the delta F508 mutation is subsequently estimated; it is found that the mutation is at least 580 generations old, but could be much older depending on the microsatellite mutation rate and the exact number of substitutions experienced in the history of the three microsatellite loci.