Familial medullary carcinoma of the thyroid (MCT) is a malignant neoplasm of the calcitonin-producing C-cells of the thyroid gland, inherited in an autosomal dominant pattern with a high degree of penetrance. We have studied 26 individuals from 5 New England families with MCT with the goal of finding a cytogenetic test useful in the detection of the presence of the gene. G-banded metaphase spreads, segregational errors of chromosomes, breakage in untreated cells, breakage in bleomycin-challenged cells, and high-resolution banding of prophase/prometaphase spreads were examined; one or more of each of these procedures were studied in at least one affected individual in each of 5 families. All specimens were coded, and slides were scored blindly. All tests were negative with respect to significant differences from controls. We did not observe the minute deletion from the short arm of chromosome 20 that was reported by other investigators to occur in individuals with multiple endocrine neoplasia types 2A and 2B which include MCT. Contrasting results from our study and several other cytogenetic studies of MCT may reflect genetic heterogeneity among the families studied and/or differing environmental factors in various geographic locations that are possibly associated with initiation and onset of the disease.