During the period from 1986 to 2000, 85 adult patients with solitary borrelial lymphocytoma were diagnosed at the Department of Infectious Diseases, University Medical Centre Ljubljana, Slovenia. There were 36 (42.4%) females and 49 (57.6%) males with a median age of 49 (15-74) years. Borrelial lymphocytoma was located on the breast (nipple--areola mammae region) in 68 (80%) patients, on the ear lobe in eight (9.4%), and in other locations in nine (10.6%). A concomitant erythema migrans enabling clinical diagnosis of Lyme borreliosis was registered or reported in 67 (78.8%) patients. Fifteen (17.6%) patients had no accompanying symptoms, 34 (40%) reported local and constitutional symptoms, 23 (27.1%) recounted only local symptoms, and 13 (15.3%) patients had solely constitutional symptoms. Clinical findings indicating early disseminated borrelial infection were observed at the first visit in 12 (14.1%) patients: six (7.1%) had multiple erythema migrans, one had meningitis, one meningoradiculitis and arthritis, one radiculoneuritis and arthritis, one peripheral facial palsy and concomitant meningitis, and two arthritis. In addition, one of the patients with borrelial lymphocytoma on the breast had acrodermatitis chronica atrophicans. A seropositive response to borrelial antigens was found in 30 (35.3%) patients at the initial examination. In 11/46 (23.9%) patients, infection with Borrelia burgdorferi sensu lato was confirmed by isolation of the agent from lymphocytoma tissue. Eight out of nine (88.9%) typed borrelial strains were found to be B. afzelii, and one (11.1%) B. bissettii. Patients were treated with doxycycline, azithromycin, amoxycillin, cefuroxime-axetil, phenoxymethylpenicillin, or ceftriaxone. Median time to complete disappearance of lymphocytoma was 28 days (range 7-270 days) after the institution of antibiotic treatment; disappearance took longer in patients with prolonged duration of the skin lesion prior to treatment. Treatment failure was registered in 11 (12.9%) patients who were later re-treated. The outcome of borrelial infection assessed at the end of a follow-up period of one year was favourable.