Colon cancer is the third most common diagnosis and the second deadliest malignancy for both sexes combined. Malignant diseases are one of the most important public health problems in the Republic of Croatia. Prevention and early detection of diseases are the most successful ways to fight against malignant diseases and contribute to reducing the number of cancer patients and mortality. A well-organized and implemented cancer screening program at the population level should be scientifically based, long term profitable and based on examples of good practice. The aim of this research is to examine the reasons for the low response to the national program for the early detection of colon cancer in the population over 50 years old in family medicine clinics in the city of Zagreb. An anonymous questionnaire was used for the research, which is available in the article "Reasons for participation and nonparticipation in colorectal cancer screening". Statistical data processing was done with the STATISTICA 11.0 software package. A total of 83 respondents were included in the research, of which 71% were women and 29% were men. The age of the respondents ranged from 50 to >70 years, with the most represented age group being between 50 and 60 years old (59%), followed by the age group from 61 to 70 years old (24%). All respondents largely or completely agree with the statement that every person who receives an invitation for screening should respond the same (4.8±0.4) as well as with the statement that the earlier treatment is started, the greater the chances of recovery ( 4.9±0.3). As many as 61% of respondents either completely disagree or disagree with the statement that they do not think screening will help prevent colon cancer (2.4±1.3), and 58% of them with the statement that they think they are healthy and that they do not need check-ups (2.2±1.0), while as many as 82% of them either completely disagree or disagree with the statement that screening for colon cancer is not significant and has no advantages" (1.9±1 ,1). The goal of cancer screening and early detection is to cure cancer by detecting malignancy or its precursors at an early stage - before symptoms appear, when cancer treatment is most effective.