We investigated the emergence of resistance to 15 anti-pseudomonal antibiotics amongst Pseudomonas aeruginosa isolates from 34 chronically colonized patients with cystic fibrosis by comparing the susceptibilities of strains isolated before 1987 and after 1989 from the same patients. Strains obtained after 1989 from a further 19 patients who were newly colonized served as controls. The 34 pairs of isolates demonstrated a marked increase in resistance which could not be accounted for by a general increase in resistance during the intervening years since the susceptibility patterns of strains isolated before 1987 were similar to those of strains isolated from patients in the control group. There was a strong correlation between this increase in resistance and both the frequency of admissions to and the number of days spent in hospital. Cluster analysis of the changes in susceptibility for individual antibiotics revealed four distinct patterns of resistance: the fluoroquinolones, with the exception of ofloxacin; the aminoglycosides; the ureidopenicillins and aztreonam; and the cephalosporins, carbapenems, carboxypenicillins and ofloxacin. We conclude that the long-term administration of anti-pseudomonal antibiotics to patients who are chronically colonized with P. aeruginosa is associated with the development of resistance.