Cy Morong publishes a list of Hall of Fame batters who hit home runs off Hall of Fame pitchers:

The other day I mentioned to a friend that I had been to a game in Montreal in 1997 and saw Chipper Jones hit a grand slam off of Pedro Martinez. As you can see from the list below, the last case was in 1995 (and the last one before that was in 1985). That ten year gap is interesting.

Maybe it has been happening less in recent years or decades as the number of teams expanded, possibly making it less likely for future Hall of Famers to face each other. Now it probably takes time for cases to make the list since it can take years for players to get into the Hall, so that might be the reason why it has not happened in twenty years. Of course, it also depends on the writers and any selection committees that choose players. Roger Clemens, for example, allowed grand slams to Mike Piazza and Frank Thomas. Without a PED issue, Clemens would be in and we would have some more recent cases.

Note that Greg Maddux did not allow a regular season grand slam until 1996, and only gave up three for his career. He did allow a slam to Will Clark during the 1989 playoffs. Mark McGwire, John Olerud, and Benito Santiago hit the homers. Without the PED stain, McGwire would be in and we would also have another example. It’s also clear it took a very good hitter to slam Maddux. Santiago was the first of the three.