Arbitrarily Primed Polymerase Chain Reaction has been used for an epidemiological evaluation of 42 strains of P. aeruginosa isolated from nine cystic fibrosis patients during a three-year investigation period. The resistance patterns of the same strains have also been evaluated. The AP-PCR type fingerprinting was perfomed with primers 10514 and 208. Resistance was evaluated by the Minimal Inhibitory Concentration method. With 10514 eleven different genotypes could be evidenced, while with 208 only five of them could be detected. During the investigation period patients were always colonised by the same genotype. A possible correlation between resistance pattern and genotype with both primers has shown, within the same patient, a correspondence of about 20% for 10514 and a correspondence of only 10% for 208. Patients are colonised by one or two strains of P. aeruginosa and there is no relation between genotype and resistance pattern.