For those who don't know, look at the org website (
http://www.usmle.org/) for facts on STEP 1. Average is 215-220 with about a 93% first time pass rate in US schools. 99 is not a percentile, it is the 2-digit score. 75 is passing, 99 is some percentage on the distribution curve of the scores for that particular test group (you are rated against peers in the same testing period). My friend who took it last year had a 99 with a 240+. My score was just released - I passed in the average range (Jun '05).
First Aid for STEP 1 and Kaplan Q-bank are invaluable study aids. You will pass if you know these well. Do much better to know them cold!