The article presents the most frequent errors made in the ultrasound diagnosis of the urinary system. They usually result from improper technique of ultrasound examination or its erroneous interpretation. Such errors are frequent effects of insufficient experience of the ultrasonographer, inadequate class of the scanner, insufficient knowledge of its operation as well as of wrong preparation of patients, their constitution, severe condition and the lack of cooperation during the examination. The reasons for misinterpretations of ultrasound images of the urinary system may lie in a large polymorphism of the kidney (defects and developmental variants) and may result from improper access to the organ as well as from the presence of artefacts. Errors may also result from the lack of knowledge concerning clinical and laboratory data. Moreover, mistakes in ultrasound diagnosis of the urinary system are frequently related to the lack of knowledge of the management algorithms and diagnostic possibilities of other imaging modalities. The paper lists errors in ultrasound diagnosis of the urinary system divided into: errors resulting from improper technique of examination, artefacts caused by incorrect preparation of patients for the examination or their constitution and errors resulting from misinterpretation of ultrasound images of the kidneys (such as their number, size, fluid spaces, pathological lesions and others), ureters and urinary bladder. Each physician performing kidney or bladder ultrasound examination should possess the knowledge of the most frequent errors and their causes which might help to avoid them.