Five cases of Roberts syndrome (RS) in four nuclear families are reported and the wide range of phenotypic variation among them is described. This is in contrast with the remarkable uniformity of the cytogenetic findings. Indirect immunofluorescence with seric antibodies from patients with CREST, revealed that the centromeric structures are normal in RS thus confirming J. German's assumption that the chromatid repulsion is confined to the heterochromatin. The authors quantified the phenomenon of centromeric heterochromatin separation (as occasionally revealed by C-bands in normal subjects) in obligate heterozygotes and possible heterozygotes for RS. The results are indicative of the possibility to screen for heterozygotes. The nosology of RS and related syndromes is discussed in view of the cytogenetic findings and the natural history of the disease.