Assessment of the efficacy of an antibiotic drug used in patients with various manifestations of dermatoborreliosis is crucial. Clinical judgement alone (resolution of the present dermatologic lesion, prevention of later major or minor sequelae) is not sufficient in erythema migrans and acrodermatitis chronica atrophicans. Thus, laboratory tests are desirable to prove the benefit of an antimicrobial agent. It was intended to establish a constant parameter--besides the clinical picture--for assessing the efficacy of antibiotic treatment in patients with dermatoborreliosis in terms of eradication of Borrelia burgdorferi from the site of infection. Polymerase chain reaction (PCR) was therefore performed from pretreatment biopsy specimens from lesional skin of 36 erythema migrans patients (m:f = 15:21, mean age 49 years) and seven acrodermatitis chronica atrophicans patients (m:f = 0:7, mean age 59 years), respectively. After antibiotic therapy with minocycline (100 mg, orally twice daily, 14 days) for erythema migrans, and ceftriaxone (2 g, intravenously once daily, 14 days) for acrodermatitis chronica atrophicans another punch biopsy was obtained and analysed by PCR. In pretreatment specimens, B. burgdorferi-specific DNA was amplified by PCR in 23/36 erythema migrans patients (69%), and in 5/7 acrodermatitis chronica atrophicans patients (71%). After antibiotic therapy, PCR yielded negative results in all of these cases. Clinically, all patients showed complete recovery or at least marked improvement of lesions at this time. PCR appears to be a reliable parameter for the assessment of the efficacy of antibiotic treatment in dermatoborreliosis.