Monitoring of Legionella bacteria is important for public health reasons in order to identify the environmental sources which can pose a risk of legionellosis, such as hot and cold water distribution systems and associated equipment. Different international standards describe how to take water samples and which technique can be used to analyze these water samples. Worst case scenario's for sampling indicate that the first liter, without a flush, should be taken to evaluate the risk. During our research we evaluated different sampling protocols. The full- scale test facility at Buildwise, contaminated with Legionella pneumophila bacteria served as case-study. Samples were taken on a regular base from the sampling valves (Depart & Return) and from the faucets (shower or kitchen faucet). The sampling ball valves on the depart and return were mounted on T- pieces. This means that a small water volume (~4 ml) from the circulation loop is trapped in the connection, presenting a small 'dead zone' suitable for biofilm development. The study included 'first liter' samples, taken at once (volume 1liter), as well as fractionised water samples (collecting the first 200 ml water separately from the 800 ml water). Also biofilm measurements were included to evaluate the contamination level of the circulation loop. This article will point out the influence of the sampling protocol on the interpretation of the 'Legionella contamination level 'within a sanitary installation. The study indicates the importance of a well described sampling strategy and protocol, in compliance with the information needed for a risk assessment. DOC_AB_NL:Monitoring of Legionella bacteria is important for public health reasons in order to identify the environmental sources which can pose a risk of legionellosis, such as hot and cold water distribution systems and associated equipment. Different international standards describe how to take water samples and which technique can be used to analyze these water samples. Worst case scenario's for sampling indicate that the first liter, without a flush, should be taken to evaluate the risk.