This article deals with the supervisory boards’ activity of Polish companies listed on the Warsaw Stock Exchange. Its purpose is to determine the relationship between firm performance measured by ROA and this activity. Poisson regression analysis conducted on a sample of 1611 observations in 356 companies between 2008 and 2015 showed that there are statistically significant and negative relationships between the ROA values measured both in the current and previous year and the supervisory boards’ meeting frequency. Thus, the results of the study confirmed that these bodies are becoming more active in the face of the deteriorating financial results of companies and are trying to be effective mechanisms for mitigating the agency's conflict.