We evaluated whether c-myc and ras P-21 oncogene expression could identify familial polyposis coli/Gardner's syndrome patients at risk for colon cancer. Monoclonal antibodies recognizing c-myc and ras P-21 proteins were used in immunohistochemistry to stain 26 paraffin-embedded tissue specimens collected over 12 years from 5 familial polyposis coli/Gardner's syndrome patients at various stages of their disease. Differences in staining intensity between specimens were noted with each of the two tissues markers; however, c-myc showed also a distinct cellular staining pattern in patients with advanced histologic features. The c-myc oncogene exhibited strong homogeneous cytoplasmic staining in all adenocarcinoma specimens; weak cytoplasmic staining was found in normal biopsies and in 5/10 familial polyposis coli/Gardner's syndrome specimens in the early stage of disease. In contrast, a heterogeneous staining pattern with a strong supranuclear and weak nuclear and cytoplasmic staining was demonstrated in specimens with advanced histologic features of familial polyposis coli/ Gardner's syndrome and also in postoperative ileal specimens. Anti-ras P-21 antibody, on the other hand, demonstrated a homogeneous cytoplasmic staining pattern in all specimens. We feel that the c-myc oncogene expression, in distinction to that of ras P-21, has potential as a genetic tissue marker to distinguish early from more progressive disease.