We studied chromosomal alterations in the peripheral blood lymphocytes of 10 individuals with colorectal polyps and 10 asymptomatic first-degree relatives of patients with colon cancer or colorectal polyps. The analysis was performed on T-lymphocytes using short term blood cultures and on B-lymphocytes by establishing lymphoblastoid cell lines by Epstein-Barr virus transformation. Chromosomal changes were not common in T- and B-lymphocytes. Chromosomes 1 and 5 were most frequently involved in numerical or structural changes in the patients with polyps as well as in the asymptomatic relatives. These alterations were observed in either the T-lymphocytes or the B-lymphocytes but rarely in both, thus accentuating the importance of studying both the cultures concurrently. Chromosome 5, which is known to play an important role in the development of adenomatous polyps, was found to be involved in 6 (60%) of 10 patients with polyps and 4 (40%) of 10 asymptomatic relatives. These findings show that lymphocytic chromosomal analysis can aid in identifying individuals who are genetically susceptible and are at a higher risk of developing colorectal cancer. Because lymphocytic chromosomal analysis is relatively simple and inexpensive, we expect that it will be very useful in screening asymptomatic individuals who are at a higher risk due to inherited or environmental factors.