Erythema migrans (EM) is the most common and recognized clinical manifestation of early Lyme Borreliosis (LB) in Europe. Purpose: to evaluate clinically and serologically all the patients with EM and to correlate the serology with clinical approach. Material and method: A prospective analyses of all patients that came with EM in our University Hospital of Infectious Diseases from Cluj-Napoca Romania between 1st of April to 31st of August 2011. We registered data regarding: age, gender, interval between tick bite and EM occurrence, interval between EM appearance and hospital presentation, serology (IgM and IgG for Borrelia burgdorferi sl) at presentation and one year after, antibiotic treatment, duration of antibiotic treatment and clinical outcome. Results: 44 patients with EM, 2 had Multiple EM, 23 male (52 %), average age 41.3 years (min 3 years, max 84 years, median 43.5 years). Serology at presentation was positive in 15 patients for IgM and in 9 patients for IgG with ELISA and for 13 for IgM and 16 for IgG with WB. At the one year follow up positive results for IgM were found in 14 patients with ELISA and in 11 patients with WB. IgG was positive in 7 patients with ELISA and in 5 with WB. The average time interval between the tick bite and EM was not significant different for patients with negative or positive IgM or IgG. The interval between EM occurrence and treatment initiation had a strong significance for IgG. After one year no statistical significance was found for positive IgM for any of the analyzed factors while for IgG positivity we found a significant importance for age and duration of incubation. No influence of antibiotic or duration of treatment was found on serology of our patients. Conclusions: Our data brings additional evidence that the serological profile is unpredictable.