Beta parameter is one of the commonly used measures of the investment risk of individual stock or portfolio. It plays a crucial role in modern portfolio theory particularly in management of financial investment portfolios. In the field of beta parameter, numerous studies have been conducted, especially beta properties stability in the context of the stock market cycle phases, measuring frequency of rate of return, and the length of a sample period. There are much fewer studies concerned beta parameter in the countries of Central and Eastern Europe which have undergone systemic transformation at the end of the previous century. From a scientific point of view, it is interesting to know how the beta parameter behaves in these countries.

The main goal of this article is to examine the beta parameter stability over bull and bear market conditions on the Warsaw Stock Exchange. The paper presents an analysis of beta stability for 134 stocks of the largest companies listed at the WSE during years 2005–2013. To verify statistically the hypothesis of beta parameter stability, we used monthly returns in the Sharpe’s single-index model. In the first part of the article, we present a brief review of the literature and methodology of the study, while in the second part, the obtained results and conclusions are shown.