Fifty-one Pseudomonas aeruginosa isolates were differentiated into 21 types by ribotyping. Several enzyme combinations, including the best ones proposed in literature, were utilized and the highest discrimination was reached by individual digestion with PvuII, HindII, and EcoRI or BamHI. Clinical isolates from outbreaks were clonally related as identified by this molecular approach. Restriction rDNA profiles were composed of strong and weak bands. Using 6 micrograms DNA we were able to demonstrate that PvuII, HindIII, and BamHI weak bands were reproducible. These weak bands should be considered not only to accomplish the highest discrimination but also to correctly assign isolate clonality. Conversely, we found that EcoRI weak bands were not reproducible and, therefore, are not recommended for ribotype analysis. Finally, profiles differing in one single band actually represented isolates of different genotype, as confirmed by further analysis using other molecular methods. In this report on P. Aeruginosa ribotyping of clinical isolates, criteria for band pattern interpretation are established.