For individuals suspected of having hematologic neoplasms, interpretation of the clinical significance of sporadic cells with chromosome breakage, structural anomalies, aneuploidy, or polyploidy is often difficult. To help resolve this problem, we established normal cytogenetic values for bone marrow (BM) by investigating 219 BM transplant (BMT) donors using standard techniques for chromosome analysis. The donors ranged in age from 2 to 58 years and were studied for 7 years. The constitutional karyotype for two individuals was 47,XXY; one was mos45,X/46,XX, one was mos46,XX/47,XX, + mar, and 215 were normal. Among other statistics, the median and normal ranges (95th percentile) were determined for any kind of chromosome abnormality, autosomal loss, autosomal gain, sex chromosome loss, sex chromosome gain, chromosome breaks or gaps, major structural abnormalities, and polyploidy. The results suggest that random loss of chromosomes is common in cytogenetic preparations of BM, appears to be largely technical and is inversely proportional to chromosome size. Cells with extra chromosomes or with structural abnormalities are rare in normal BM. No specific sporadic structural abnormalities of chromosomes are associated with normal BM. The widely accepted cytogenetic definition for an abnormal clone appears to be valid, with the possible exception of occasional studies involving loss of smaller autosomes. There may be a correlation between loss of the Y chromosome and age of the patient.