Adults with CF followed in a university center were assessed for the presence of the most common CF gene mutation, delta-F508. Excluding one member of a sibling pair, 29 of 55 subjects had two copies of delta-F508 (homozygotes), 23 had one copy of delta-F508 with the other CF mutation not identified (complex heterozygotes) and three were lacking delta-F508. A wide range of clinical severity was seen among individuals carrying two copies of the delta-F508 gene, who are genetically identical at the CF gene locus. The number of individuals diagnosed with CF as adults was significantly lower in the homozygote group (1 of 29) as compared with the heterozygote group (7 of 24). No differences were detected between groups in pulmonary function, non-pulmonary complications or overall clinical severity. These results suggest that environmental or background genetic factors contribute significantly to the variability in pulmonary and other complications seen among individuals with CF.