Parasitic infections of the digestive tract remain an important health problem on a global scale. The current epidemiological situation in Poland is unknown because most parasitic diseases, including all caused by helminths, are not subject to reporting. Clinical practice shows that diseases of such etiology are sometimes overlooked in the process of differential diagnosis. In addition, the commonly available classical examination of a stool sample utilizing a light microscope is a relatively low-sensitivity test. This study presents a case report of a 46-year-old patient admitted to the Clinic for planned diagnostics due to weight loss without other gastrointestinal symptoms. In the course of the diagnosis, it occurred that the cause of the weight loss was a parasitic infestation of the gastrointestinal tract with roundworms, which were observed in a colonoscopy, but no parasites were found in the collected material in stool examinations performed three times. The presented case report demostrates that parasitic diseases still should be taken into consideration in clinical practice in the process of differential diagnosis. In addition, further effort is needed to improve the sensitivity of parasitological diagnostic methods. Moreover, there is a need for research on the current epidemiological situation in Poland in the field of gastrointestinal parasitosis.