The purpose of this study is to examine market efficiency on the Stockholm Stock Exchange by looking into whether investors react to new information in the form of quarterly reports and annual reports. The study aims to determine which type of these reports investors value the most as a basis for decision-making, and therefore which one contributes the most to an efficient market. To address these questions, this study uses an event study methodology on the stocks within the OMXS30 index during the period 2021-2024, encompassing 348 quarterly reports and 87 annual reports. Abnormal returns were calculated and aggregated to identify market reactions upon the release of these reports. The results show that both annual reports and quarterly reports give rise to abnormal returns, indicating that the market reacts to the information presented in these reports. Although both types of reports contribute to an efficient market on the Stockholm Stock Exchange, the study could not determine which type of report is valued most by investors at a statistically significant level. However, the results suggest that annual reports have a slightly higher cumulative aggregated abnormal return compared to quarterly reports. This study contributes to the existing research on market efficiency and financial reporting on the Swedish stock market and suggests further research with a larger sample to more clearly determine which type of report most affects market efficiency