Codon 12 and 13 mutations in 170 colorectal cancer (CRC) and 66 gastric cancer (GC) specimens were analysed by an 'enriched' polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. All identified mutations were verified by direct sequencing of the second PCR products. Among the 170 CRC specimens, mutations were identified in 47 (28%) and 13 (7.6%) cases in codons 12 and 13, respectively. In the 66 GC specimens examined, however, mutations in codons 12 and 13 were only detected in two (3.0%) and one (1.5%) cases, respectively. Mutations in both codon 12 and 13 were found in 3/170 (1.8%) CRCs and 1/66 (1.5%) GCs. Duplicate mutations were never identified in the same allele, which was confirmed by direct sequencing of the second amplified products. The majority of colorectal and gastric cancer cells with KRAS mutations are homogeneous because they have the same KRAS mutation. A few colorectal or gastric cancers, however, showed heterogeneity, as verified by the fact that single mutations were identified in the same allele.