An infant is reported who presented with a de novo 21;21 translocation trisomy 21 and an atypical phenotype for Down syndrome (DS). Findings included microcephaly, small stature, downslanting palpebral fissures, absent Brushfield spots, moderate micrognathia, left ptosis, left torticollis, severe developmental delay, seizures, and hypertonia. Further clinical evaluation using both the diagnostic criteria for DS and the Jackson checklist of 25 signs was inconsistent with the diagnosis for DS. Blood karyotype revealed: 46,XX,+21,dic(21;21) (p11.2;p11.2). Fluorescence in situ hybridization (FISH) analysis confirmed the trisomy 21 translocation. Both parents had normal karyotypes. Chromosome and FISH analyses were performed on skin fibroblasts. These studies revealed mosaicism for a translocation trisomy 21 cell line as wel as a second cell line consisting of one normal chromosome 21 and a ring chromosome 21 derived from translocation 21q21q which appeared to have a deletion of the critical region for DS involving the distal portion of the thelong arm of chromosome 21. The chromosome findings illustrate an atypical phenotype in the spectrum of mosaic DS and suggest possible mechanisms for the variability of the phenotype. It also emphasizes the importance of evaluating other tissues for mosaicism when presented with atypical clinical findings.