A segregation analysis has been carried out for 11 families with trisomy 10p caused by familial translocations involving a segment of the short arm of chromosome 10. The theoretical basis for the analysis is considered in some detail. No differences were found between the segregation pattern in the offspring of carrier mothers and that of carrier fathers. There was a high risk for offspring with trisomy 10p (22%). A phenotypically normal descendant also has a high risk of becoming a balanced translocation carrier (71%). This result does not deviate significantly from the theoretical value of 50%.