After a steep decline during the second quarter of 2009, the relative strength spread has been flat for the majority of the last year. The historical tendency has been for relative strength to do especially well as bull markets move past the initial move off the bear market lows. I suspect that this could be transitioning into an increasingly favorable environment for relative strength investing.
The chart below is the spread between the relative strength leaders and relative strength laggards (universe of mid and large cap stocks). When the chart is rising, relative strength leaders are performing better than relative strength laggards. As of 5/10/2010: