From 1978 to 1988 strains of gentamicin-susceptible (Gms) and gentamicin-resistant (Gmr) Klebsiella pneumoniae were saved from annual surveillance cultures of the perineal region of patients with spinal cord injury (SCI). Of 38 strains selected for further study (24 Gms and 14 Gmr), there were 23 different serotypes (two nontypable). Fourteen Gms as well as 14 Gmr strains displayed no common plasmid patterns, but all contained a large plasmid of 168-208 kb. Among the 14 Gmr strains, nine had large conjugative plasmids of approximately the same size (166-193 kb), which conferred to a susceptible Escherichia coli host an identical resistance pattern: ampicillin, chloramphenicol, gentamicin, piperacillin, trimethoprim-sulfamethoxazole, tetracycline, and tobramycin. Of the nine transconjugants, eight contained a single plasmid. One transconjugant contained a 168- and 80-kb plasmid. Restriction endonuclease digestion patterns of the R-plasmids revealed minimal similarity. We conclude that, during a 10-year period, different large R-plasmids have spread among multiple serotypes of K. pneumoniae in spinal cord injury (SCI) patients in one rehabilitation hospital. We hypothesize that other genes located on large, R-, and non-R-plasmids may confer an additional advantage for colonization by K. pneumoniae in SCI patients.