Rubinstein-Taybi syndrome (RTS) is a dominant Mendelian disorder characterised by mental retardation, a typical facies, broad thumbs and short stature. Previous reports indicated that 4-25% of RTS patients have a submicroscopic 16p13.3 deletion of the CBP gene. Using FISH and cosmid probes RT100, RT191 and RT203 we studied 45 RTS patients from Germany, the Czech Republic, Austria and Turkey and found four deletions (8.9%, pooled data including other studies: 11%). All deletions were interstitial; three spanned the CBP gene (RT100-RT203) and one was smaller (RT100 only). Previous studies reported no phenotype-genotype correlation between RTS patients with or without a deletion. Our findings suggest a more severe phenotype. The mean age at presentation was 0.96 years in patients with a deletion as against 11.12 years in those without. Patients A and B with a deletion died in infancy which is rare in RTS and was not observed among the other patients. Patients A and D had accessory spleens, Patient A with hypoplastic left heart, abnormal pulmonary lobulation and renal agenesis. This is the second report of hypoplastic left heart and the first report of polysplenia with RTS. The signs suggest a developmental field defect (disturbance of laterality) either as a newly recognised pattern of RTS, or alternatively a novel contiguous gene syndrome.