Understanding what determines the stability and global spread of pneumococcal clones is important for future public health interventions. This requires better knowledge about the stability and distribution of existing clones. In this study we characterized and compared penicillin nonsusceptible (PNSP) and susceptible pneumococci (PSP) from southern Sweden. A total of 166 isolates of Streptococcus pneumoniae, recovered in Malmohus County between 1982 and 1997, were analyzed with molecular and microbiological techniques; 107 PNSP isolates of serogroup 15, collected between 1992 and 1995, and 15 PNSP isolates of serogroup 9, isolated in 1996 and 1997, were studied. In addition, PSP of serogroups 9 and 15, isolated approximately 10 years apart, were studied; 23 of serogroup 9 and 21 of serogroup 15, isolated in 1982-1983 and 1992-1993. As expected, a high degree of homogeneity was found in the PNSP isolates, where all the isolates of serogroup 9 belonged to the same clone, Spain(9V)-3, and the majority of the serogroup 15 isolates belonged to the Sweden(15A)-25 clone. The remaining PNSP isolates of serogroup 15 belonged to a clone found in The Netherlands and Greece. An unexpectedly high degree of clonality and stability of the PSP isolates was observed. Isolates representing clones that remained stable for over 10 years were found among both serogroups. These results indicate that factors, other than antimicrobial resistance, play an important part in establishing successful clones of S. pneumoniae, and these factors may be instrumental in determining the success or failure of clones after they acquire resistance.