Over the last twenty years, many researchers have documented that average rate of return from stocks in the month of January is higher than in any other month of the year. More recently, several researchers have offered a convincing explanation for this so called January effect. Tile window dressing hypothesis claims that high returns on risky securities in January are caused systematic shifts in portfolio holdings of institutional investors at the turn of the year. The purpose of this paper is to provide some insights into this seasonal behaviour of stock prices by testing the window hypothesis and examining the impact mutual funds trading has had on the aggregate stock market in Portugal over the 1996-2001 period,