The efficiency of the market for stock index futures and profitability of arbitrage for contracts on the Warsaw Stock Exchange Index WIG20 is studied in this paper. The Polish market has unique attributes: in a relatively short time the risk-free interest rate has decreased significantly, short sale cannot be used to construct an arbitrage position by institutional investors, and the dividends are small and paid in an irregular manner. Examining intraday transaction data shows that ex post and ex ante violations for short arbitrage reveal almost all properties of a mature market. Nonetheless, findings for long arbitrage indicate inefficiency of the market.