BACKGROUND: Bloodstream invasion in Lyme disease has been difficult to study because until recently blood culture methods were too insensitive to detect spirochetemia. OBJECTIVE: To evaluate the clinical and laboratory features of spirochetemic patients. DESIGN: Cross-sectional study. SETTING: Lyme Disease Diagnostic Center in Valhalla, New York, 1997 to 2002. PATIENTS: 213 untreated adults with erythema migrans. INTERVENTION: Blood culture for Borrelia burgdorferi. MEASUREMENTS: Symptom scores and selected laboratory measures. RESULTS: Spirochetemia was found in 93 (43.7%) patients. Spirochetemic patients were more often symptomatic (89.2% vs. 74.2%; P = 0.006) and more often had multiple erythema migrans lesions (41.9% vs. 15.0%; P < 0.001) than patients without spirochetemia. However, 8 (22.9%) of the 35 asymptomatic patients with a single skin lesion nevertheless had a positive blood culture. Risk for spirochetemia was present the day the patient noticed the lesion and continued for more than 2 weeks. LIMITATIONS: Long-term outcome data were not available. CONCLUSIONS: The high rate, early onset, and prolonged duration of risk for spirochetemia explain why untreated patients with erythema migrans are at risk for dissemination of B. burgdorferi to anatomic sites beyond the lesion site. Differences in the strain of the infecting spirochete, as well as host factors, may be important determinants of hematogenous dissemination.