A microbiological analysis of 102 patients suffering from cystic fibrosis was conducted over a 22 month period. 20 microbial species with the following incidence were identified: Pseudomonas aeruginosa: 83.4%; Candida albicans: 29.4%; Staphylococcus aureus: 24.5%; Staphylococcus epidermidis: 11.8%; Haemophilus influenzae: 11.8%; Streptococcus pneumoniae; 6.9%; Pseudomonas maltophilia: 6.8%; Aspergillus fumigatus: 5.9%. Other species were present in less than 5% of the patients. In the majority of specimens with P. aeruginosa, more than one type (up to six) was detectable. These strains were identical in colony appearance, O-serotype and pyocin-type. Quantitative analysis revealed concentrations of colony-forming units of 10 (7) to 10 (9) for P. aeruginosa, 10 (6) to 10 (8) for P. maltophilia, 10 (4) to 10 (7) for S. aureus, 10 (4) to 10 (6) for S. epidermidis and 10 (4) to 10 (7) for C. albicans in the majority of specimens. Significant differences were observed in the time periods during which the pathogens persisted in the patients. Maximum persistence was observed for P. aeruginosa. P. maltophilia and A. fumigatus had about similar persistence rates, which were lower than those for P. aeruginosa but above those for S. aureus and H. influenzae. S. epidermidis was eliminated within shorter periods than S. aureus. C. albicans, although the second most frequent microorganism identified, showed a very low persistence rate. The microbiological analysis confirms results from other research centers (high incidence of P. aeruginosa), but reveals significant regional differences as well (Pseudomonas cepacia not detectable, higher incidence of P. maltophilia and C. albicans). This underlines the necessity for detailed qualitative and quantitative microbiological analysis of sputa from cystic fibrosis patients as a prerequisite for rational analysis of etiological, epidemiological and therapeutical aspects of cystic fibrosis.