If you click on the chart above, you can see the updated count of 10-day new highs minus lows among the 40 stocks that I track in my S&P 500 basket. The 40 stocks are evenly divided among the Materials, Industrials, Consumer Discretionary, Consumer Staples, Energy, Health Care, Financial, and Technology sectors. They include the most highly weighted stocks within the various S&P 500 sector indexes.
You can see clearly that new highs have consistently expanded during the recent market rally. Interestingly, going back to the start of 2004 (N = 908 trading days) when new highs have exceeded new lows by more than 20 issues (N = 30), the next ten days in SPY have averaged a loss of -.56% (14 up, 16 down). That is considerably weaker than the average 10 day gain in SPY for the remainder of the sample of .32%.
This suggests that, by the time a solid majority of issues are making new short-term highs, the buyers have generally gotten their business done and there has been no bullish edge in the near term.