OBJECTIVES: The aim of this study is to answer the following questions: 1) How high is the self-reported nonadherence (NA) of German patients with the need to regularly take medication? and 2) Which factors capable of explaining this self-reported NA can be identified by multivariate analysis? METHODS: Two cross-sectional surveys (phone survey with 1177 patients; face-to-face in-depth survey with 340 patients in 17 German pharmacies) were conducted. Self-reported NA was measured by the generic Morisky scale (either as 4 items or 8 item MMAS). Identification of explanatory factors was conducted on the basis of multivariate logistic regression analysis (including the calculation of additive risks by dichotomization of significant explanatory factors). RESULTS: 1) Approximately 35-40 % of the patients can be described as non-adherent (38.8 %/35.3 %); 2a) Survey 1: Only a few socio-demographic factors are able to explain the NA (chronic disease, some aspects of age, and low number of required medications to take); and 2b) Survey 2: Most results of the first survey can be replicated. However, intentional NA explanations have considerably more influence: positive medication belief, a positive mood, and a good patient-doctor relationship reduce the NA risk. Furthermore, patients who are easily able to recognize the correct medication on the basis of the identification of the packaging have a significantly reduced NA probability. When additive risk is considered, patients who are chronically ill but display no other risk factors have an NA probability rate of 10.4 %, for patients displaying all identified risk factors this rate increases to 93.9 %. CONCLUSIONS: Our surveys are the largest and most detailed to have been conducted in Germany concerned with the theme of medication-based NA. Our results show that approximately one-third of patients can be classified as non-adherent. Intentional NA factors explain the NA considerably better than do socio-economic factors.