Despite rather strict recommendations for antibiotic treatment of disseminated Lyme borreliosis (LB), evidence-based studies on the duration of antibiotic treatment are scarce. The aim of this multicenter study was to determine whether initial treatment with intravenous ceftriaxone (CRO) for 3 weeks should be extended with a period of adjunct oral antibiotic therapy. A total of 152 consecutive patients with LB were randomized in a double-blind fashion to receive either amoxicillin (AMOX) 1 g or placebo (PBO) twice daily for 100 days. Both groups received an initial treatment of intravenous CRO 2 g daily for 3 weeks, followed by the randomized drug or PBO. The outcome was evaluated using the visual analogue scale at the follow-up visits. The final analysis included 145 patients, of whom 73 received AMOX and 72 PBO. Diagnoses of LB were categorized as either definite or possible, on the basis of symptoms, signs, and laboratory results. The diagnosis was definite in 52 of the 73 (71.2%) AMOX-treated patients and in 54 of the 72 (75%) PBO patients. Of the patients with definite diagnoses, 62 had neuroborreliosis, 45 arthritis or other musculoskeletal manifestations, and 4 other manifestations of LB. As judged by the visual analogue scale and patient records, the outcome after a 1-year follow-up period was excellent or good in 114 (78.6%) patients, controversial in 14 (9.7%) patients, and poor in 17 (11.7%) patients. In patients with definite LB, the outcome was excellent or good in 49 (92.5%) AMOX-treated patients and 47 (87.0%) PBO patients and poor in 3 (5.7%) AMOX-treated patients and 6 (11.1%) PBO patients (difference nonsignificant, p = 0.49). Twelve months after the end of intravenous antibiotic therapy, the levels of antibodies against Borrelia burgdorferi were markedly decreased in 50% of the patients with definite LB in both groups. The results indicate that oral adjunct antibiotics are not justified in the treatment of patients with disseminated LB who initially receive intravenous CRO for 3 weeks. The clinical outcome cannot be evaluated at the completion of intravenous antibiotic treatment but rather 6–12 months afterwards. In patients with chronic post-treatment symptoms, persistent positive levels of antibodies do not seem to provide any useful information for further care of the patient.