Background: Due to the raised public awareness of Lyme Borreliosis (LB), its increased incidence and the increased availability of serological tests, the demand for diagnostic testing on LB has increased. This may affect the diagnostic behaviour of general practitioners (GPs). Aim of our study was to describe GPs' diagnostic behaviour when suspecting LB. Methods: In this descriptive study from January 2010 to June 2015, we used the anonymized electronic medical records of 56,996 patients registered in 12 general practices in Amsterdam, The Netherlands. The target population was identified by means of an extensive search strategy, based on International Classification of Primary Care (ICPC-1) codes, free text and diagnostic test codes. All contacts related to LB were included in the analysis. Results: 2311 patients were included, accounting for 3861 LB contacts and 2619 LB episodes. The distribution of LB contacts showed annual peaks during spring and summer. Serological testing was performed in 36.4% of LB episodes and was mostly requested in patients presenting with general symptoms (71.4%). Unnecessary testing often occurred and only 5.9% of the tests turned out to be positive by immunoblot. From January 2010 to June 2015, no significant differences were found in the number of requested serological tests. The level of serological testing during LB episodes differed significantly between the general practices (19.2% to 75.8%). Conclusions: Contrary to clinical guidelines, GPs regularly requested serology even when there was a low suspicion of LB. The development of an easy-to-use diagnostic algorithm may decrease overuse of diagnostic tests and thereby reduce overtreatment of LB.