The molecular cytogenetic characterization and clinical details of 20 patients with marker chromosomes are presented. These 20 patients, together with another 22 patients previously published, represent a cohort in which the chromosomal origin of the marker chromosomes was successfully determined in all but one case. Examination of the pooled data suggests that the satellited markers derived from chromosomes 14, 15 (when metacentric or submetacentric), those whose origin is either 13 or 21, and those small ring autosomal markers derived from both alphoid and satellite II or III pericentric heterochromatin of chromosomes 1, 9, 15, and 16 are all associated with a low risk of phenotypic abnormality. The markers identified as i(18p), ring chromosomes derived from various autosomes, and satellited markers derived from chromosome 22 are associated with a high risk of phenotypic abnormality. The phenotype of patients with acrocentric markers derived from chromosome 15 was equivocal, perhaps as a result of imprinting. Additional data are required to confirm these trends. The mild mental retardation and abnormal face of a patient with a small ring chromosome derived from chromosome 4 are described. Identification of patients with small rings originating from particular chromosomes may allow the recognition of new syndromes.