Many organizations choose business sustainability to meet environmental, social, and financial demands to ensure responsible and long-term success. In order to achieve this, enterprises have to fully and optimally apply their resources. Intellectual capital is one of their most important resources creating market value and reinforcing sustainable competitive advantages. The main goal of the paper is to indicate which elements of the intellectual capital of small and medium-sized enterprises in Poland have the greatest impact on their sustainable development in the opinions of managers. The study took into account 1041 economic entities. Managers of small and medium enterprises operating in Poland have been asked to assess the elements of intellectual capital. The paper utilizes a new approach to the importance assessment of the impact of each characteristic on business sustainability. It is based on intuitionistic fuzzy sets. The proposed method is especially useful when variables are measured on an ordinal scale. In accordance with the classical theory of measurement, it does not allow the majority of arithmetic calculations to be performed. Based on the results of the research it has been determined that the most important factors are those which constitute human capital. Moreover, the observation has been made that there is a high degree of coherence of opinions expressed by managers of small and medium enterprises. The results of the research can be applied by both managers of enterprises in managing intellectual capital and employees of companies, by increasing their awareness of the topic of the influence of intellectual capital on business sustainability.Sustainability 2020, 12, 1545 2 of 23 be equal in terms of priorities to financial aspects. Meeting economic, social and environmental goals can bring a number of benefits and increase competitiveness [1,2].Intellectual capital is one of the most valuable resources of an enterprise, which enables its sustainable development. It is comprised of human capital, structural capital and relational capital.The main goal of the paper is to indicate which elements of the intellectual capital of small and medium-sized enterprises in Poland have the greatest impact on their sustainable development in the opinions of managers. The analysis utilizes the results of a study on managing intellectual capital by enterprises in Poland. The research was conducted in May, 2019. The research was realized through the CATI (computer-assisted telephone interviewing) and CAWI (computer-assisted web interviewing) methods in a representative group of 1041 enterprises operating in Poland (employing at least 10 employees) with an assumed level of trust of 0.95 and an estimation error of 3%. It was based on a proportional selection of enterprises based on voivodeships and their size measured by the number of employees. The assumed structure of the test group also took into account the differentiation of enterprises based on their type of activity (the number of interviews conducted was ...