This study was made to determine whether and to what extent bacteremia occurred after toothbrushing in patients undergoing orthodontic treatment with fixed appliances. Twenty patients were selected, all with negative history of heart or hematologic disorders. These patients had not taken antibiotics or had a history of a cold in the previous 30 days. Blood samples of 20 ml were drawn before and 5 minutes after brushing. The immune status of the patients was tested by measurement of isohemagglutins and immunoglobulin levels. Blood samples were incubated in paired culture bottles containing trypticase soy broth (TSB) with an agar paddle and Columbia broth. All samples taken before brushing were negative for bacteria. Five of the 20 patients (25% of the sample) had positive blood tests after brushing. Both anaerobic and aerobic bacteria were identified from the blood samples. Those patients who were found to have a bacteremia did not display poor oral hygiene.