Small supernumerary marker chromosomes of 3 patients were characterized at the molecular cytogenetic level. Two ring chromosomes and one metacentric marker were shown to be distamycinA/DAPI-negative and did not possess satellite regions after conventional banding techniques. Fluorescence in situ hybridization (FISH) was performed and in all 3 cases the supernumerary markers were shown to be derived from chromosome 20. Phenotypes are described and discussed with respect to karyotypes. Two of the patients are developmentally and/or phenotypically normal. The first patient has a ring chromosome, containing a small amount of euchromatic material; the second patient is the carrier of a small, metacentric and most probably heterochromatic marker. Patient 3 has physical anomalies, including a congenital heart defect and delayed motor development, but is intellectually almost normal. His marker chromosome is a ring containing a small amount of euchromatic material.