Two hundred seventy-seven patients with chronic Lyme disease were treated with tetracycline for 1 to 11 months (mean, 4 months); the outcomes for these patients were generally good. Overall, 20% of the patients were cured; 70% of the patients' conditions improved, and treatment failed for 10% of the patients. Improvement frequently did not take place for several weeks; after 2 months of treatment, 33% of the patients' conditions were significantly improved (degree of improvement, 75%-100%), and after 3 months of treatment, 61% of the patients' conditions were significantly improved. Treatment outcomes for seronegative patients (20% of all patients) were similar to those for seropositive patients. Western immunoblotting showed reactions to one or more Borrelia burgdorferi-specific proteins for 65% of the patients for whom enzyme-linked immunosorbent assays were negative. Whereas age, sex, and prior erythema migrans were not correlated with better or worse treatment outcomes, a history of longer duration of symptoms or antibiotic treatment was associated with longer treatment times to achieve improvement and cure. These results support the use of longer courses of treatment in the management of patients with chronic Lyme disease. Controlled trials need to be conducted to validate these observations.