Streptococcus pneumoniae (SPN) is the most common cause of invasive infections in children, with high levels of mortality in developing countries. An increase in frequency of penicillin-resistant strains is reported in most parts of the world. A study was undertaken in Argentina and 5 other countries of the region, to determine the type distribution and penicillin resistance rate of SPN isolated from invasive infections in children less than 5 years old. Between June 1994 and March 1996, a total of 505 SPN isolated from sterile sites were collected from 15 hospitals located in 9 cities of different geographic areas. Clinical and epidemiological data from 443 children were analyzed. Sixty five percent SPN were isolated from children less than 2 years old. Pneumonia was the clinical diagnosis in 58% of the cases, meningitis in 22%, and sepsis in 10.6%. Isolates were recovered from blood (51.2%), pleural fluid (22.7%), CSF (20.7%), and other sterile sites (5.4%). Thirty different pneumococcal capsular types were identified and the 10 most frequent in descending order were: 14, 5, 1, 6A/6B, 7F, 9V, 19F, 19A, 16F y 23F, representing 89.3% of the total. Overall, 13.1% of isolates showed intermediate resistance to penicillin while 11.3% showed high resistance. Lethality was 8.8%, without correlation with penicillin-resistance and/or type. These result should be used in selecting the optimal combination of specific types for a conjugate vaccine, useful in children less than 2 years old and for considering therapeutic strategies for invasive pneumococcal infections.