An external audit is an audit performed by certified auditors who are not employed or have any affiliation with the company they are auditing. After the audit, the auditor is required to write an audit report in which he will express his opinion on whether the financial statements are presented fairly and in accordance with auditing standards. The auditor may give a positive opinion and a modified opinion. The auditor will give a positive opinion if he considers that the financial statements are presented fairly and in accordance with the standards, while a modified opinion will be given if he finds certain irregularities in the financial statements. The auditor may also highlight the issue of the indefinite nature of the business in the financial statements if it considers that it is compromised for some reason. The aim of this paper was to investigate whether the duration of the audit engagement affects the type of opinion the auditor gives to the audited entity. Longer engagement of auditors leads to greater acquaintance between the auditor and the manager, which can potentially lead to a decrease in the independence of the auditor and giving a biased opinion. In this research, a sample of 110 companies listed on the Zagreb Stock Exchange was examined. The results of the research showed that there is no statistically significant correlation between the duration of the engagement and the audit opinion. In addition to the type of opinion, the paper examined whether there is a statistically significant relationship between the duration of the audit opinion and the auditor's emphasis on the issue of indefinite business. The research showed that there is no statistically significant correlation. In Croatia, there is an oligopoly among auditing companies called the "Big Four", which consists of the four largest auditing companies in the world, such as: Deloitte, PWC, EY and KPMG. The paper examined whether the duration of the so-called engagement. Big four longer than the engagement of other audit firms in Croatia, and the research showed that the engagement of other audit firms is longer.