Increased antibody levels against several bacteria were found in 15 of the 34 stroke patients (44%) under the age of 45 years, but in only six of the 68 controls (9%) (p less than 0.001). Based on the serologic data, the most common preceding infections were streptococcal, staphylococcal, and enterobacterial. There was no relationship between viral antibodies and cerebral infarction. When the clinical history of the patients was also taken into consideration, stroke was found to be associated with a recent infection in 68% of the patients, as compared with 26% for the second most common risk factor, ethanol intoxication. The results suggest an association between cerebral infarction and bacterial infections in young adults.