OBJECTIVES: In current guidelines, extended thromboprophylaxis after a hip and knee replacement surgery is recommended. Based on the largest empirical survey to date dealing with the question of LMWH (low molecular weight heparins) non-adherence (NA), this study aims to deal with two main topics: (1) the study assesses whether there is any quantitative relevance of outpatient thromboprophylaxis in Germany; and (2) the degree of any non-adherence of patients in LMWH outpatient prophylaxis is assessed. METHODS: For data collection we performed structured telephone interviews with 1415 hip and knee replacement patients. Non-adherence was determined by syringe count and by indirect and direct questions to patients. All in all, 6 different non-adherence indicators based on patient interviews were defined. For multivariate analysis of factors explaining patients`non-adherence (based on WHO recommendations), we estimated 3 different logistic regressions models. RESULTS: For about 90% of patients, outpatient prophylaxis is of relevance in Germany. Non-adherence rates in LMWH outpatient prophylaxis range between 13% and 21% depending on the indicator used for measurement. Non-adherent patients miss between 38-53% of all their outpatient LMWH injections. If patients attend an outpatient rehabilitation program, the probability of their non-adherence increases significantly. Moreover, the non-adherence probability increases with each additional day of bridging (days between acute hospitalization and start of rehabilitation). Non-adherence is lower in patients who fear any thrombosis or who believe that antithrombotic drugs are the most important measure in thromboprophylaxis. CONCLUSIONS: Non-adherence in outpatient thromboprophylaxis seems to be a serious challenge. It can also be assumed that outpatient prophylaxis in other countries is at least as important as it is in Germany, for the simple reason that most other countries do not have a comprehensive network of inpatient rehabilitation clinics.