Bacterial DNA polymorphism was used to document the occurrence of three separate episodes of meningitis caused by Escherichia coli K1 in an infant. The methods employed included determination of the restriction fragment length polymorphism of total DNA and of ribosomal DNA regions as well as DNA fingerprinting by the arbitrarily primed polymerase chain reaction. By these three genotypic approaches, the three isolates obtained from the infant's cerebrospinal fluid on days 9, 34, and 70, respectively, were found to share the same patterns, which were different from the patterns of control strains. Thus, these three episodes of E. coli K1 meningitis were due to a single strain. DNA-based typing techniques seem extremely promising as tools to be used in unraveling the complex mechanisms of recurrent meningitis.