The aim of this paper is to analyze the effectiveness of the audit committees of all companies whose shares are traded on the Zagreb Stock Exchange. The survey was conducted on a sample of listed companies in 2018 which are required to complete and submit the Annual Questionnaire of the Corporate Governance Code. The questions from the questionnaire related to the audit committee were analyzed. Based on the sample, the Audit Committee Performance Index (IURO) was created and it was concluded that 55.8% of companies have high or complete audit committee efficiency i.e. that the audit committees of Croatian listed companies are highly or fully efficient. Furthemore, the relationship between the Index and the financial performance of listed companies were examined. It was found that there is a positive relationship between the Index and the size of the company, and there is no interdependence between the performance of the audit committee measured by the Index and return on assets (ROA), return on equity (ROE) and leverage. Finally, the interdependence of the Index and the audit opinion as well as the selected audit firm was examined. It was concluded that an effective audit comittee affects the successful operation of the company and therefore the audit opinion will be positive. On the other hand, no interdependence was found between the Index and the company that audited the financial statements.