Two hundred thirty randomly collected primary colorectal tumors were initially screened for microsatellite instability (MSI) with three highly informative microsatellite markers (BAT26, D2S123 and D5S346). Forty one (17.8%) tumors showed alterations in at least one marker. In further MSI analysis of these 41 MSI tumors with additional 9 markers, 21 tumors (9.6% of 230 analyzed) exhibited MSI at more than 40% and the rest 20 (8.7% of 230 analyzed) tumors exhibited MSI at 8%-20% tested markers. These results support classification of MSI tumors into high MSI tumors (more than 40% unstable loci) and low MSI tumors (less than 20% unstable loci). Based on our results the combination of BAT26 and two out of four other highly informative markers (D2S123, D5S346, BAT25 or BAT40) is recommended for rapid and reliable assessment of high MSI tumors.