The paper investigates the empirical verification of the efficacy of investment diversification using the main stock exchange indices in the Eurozone countries and Bitcoin. The paper also investigates whether and to what extent it is desirable for institutional investors, in addition to traditional financial instruments, to invest in Bitcoin. The aim of the research is to examine whether it is justified and to what extent to include Bitcoin in the portfolio of an institutional investor. Through this research, an attempt is made to find an answer to the research question: “What share of Bitcoin in the portfolio structure is justified, taking into account the ratio of return and risk”? The analysis includes data on the daily movement of selected action indices as well as the movement of Bitcoin. The methodology involves the analysis of high-frequency data, given that daily trading data were used. The results show that it is justified to include Bitcoin in the portfolio structure. Also, the results show which share of Bitcoin in the portfolio is justified from the aspect of institutional investors. The data used in the analysis cover the period from 2019 and 2020. Two portfolios have been created, one without Bitcoin and the other with Bitcoin. The goal in optimization for both portfolios is to minimize risk. The observed period of the analysis is characterized by the crisis caused by the coronary virus pandemic and the period of active bitcoin trading. The results of the research show that Bitcoin is a good source of diversification in a portfolio that contains traditional financial instruments, both for an investor who is not prone to risk, and for those investors who have a greater appetite for risk. The conclusion is that the rational behavior of institutional investors requires consideration of investing in Bitcoin using the Markowitz model. However, given the high degree of volatility, investors should be careful when making decisions about including Bitcoin in their investment portfolio. Bitcoin is an extremely volatile instrument. Given that it is a speculative and highly volatile financial instrument, investors have different views on Bitcoin. First in terms of defining this cryptocurrency and then in terms of including this instrument in the investment portfolio. By including Bitcoin in the investment portfolio, the goal of diversification has been achieved. This is to reduce the risk of the institutional investor to a minimum. In practice, this means that it is possible to create a portfolio that carries an acceptable level of risk with the desired level of return. Given that Bitcoin is an extremely volatile and consequently - risky instrument, the expected return is also - high. The results of the research show that the cryptocurrency Bitcoin can serve as a desirable instrument for diversification of the investment portfolio when looking at a portfolio that includes stock indices. The results suggest that it is desirable to include in the structure of the portfolio a certain share of Bitcoin, about 6%.