Introduction. An effective cellular immune response involves the host's ability to recognize and respond to a variety of antigens. The third complementary region (CDR3) of the T-cell receptor (TCR) is the site of antigen recognition. Recombination events within the genetic elements coding for CDR3 lead to the generation of diversity for specific antigen recognition. These recombinations result in different sizes of the mRNA/cDNA encoding CDR3. The preterm infant is unusually susceptible to certain T-cell mediated infections. The ability of the preterm infant's TCR repertoire to diversify has not been previously characterized. Objective. To describe the developmental changes of T-cell receptor diversity generation during gestation. Design. Reverse transcriptase/polymerase chain reaction was performed on umbilical cord or peripheral blood lymphocytes using a constant region primer and 26 Vβ family specific primers. The final PCR products were analyzed on a 5% acryalmide gel. Each V-specific amplification results in a number of bands representing rearrangements of the TCR Vβ chain gene segments. Analysis of the absolute CDR3 size was accomplished with an oligonucleotide primer directed against a conserved region of the 26 Vβ families and a constant region primer by PCR amplification. Results. The CDR3 length from 25 week gestation neonates was six base pairs (two amino acids) shorter than corresponding CDR3 length of adults. Term gestation neonates had CDR3 lengths which were three base pairs (one amino acid) shorter than those of adults. Samples from 24-28 week gestation had no limitation of Vβ family expression. Oligloclonal expansion was more common in infants as compared to adults (p=0.03). Beyond 29 weeks gestation most infants had more than one oligoclonal expansion with concomitant restricted CDR3 expression within that Vβ family. Conclusions. T-cell receptors undergo maturation during gestation and postnatally which is reflected by a lengthening of the antigen binding region of the TCR. The naive repertoire found in most infants 24-28 weeks gestation appears diverse; however, in some infants oligoclonal expansions and marked restrictions of other CDR3s within the same Vβ family may be due to in utero antigenic challenge. Limitation of T-cell repertoire in response to antigenic challenge differs from that of the adult. We speculate that the shorter CDR3 region and unusual response to antigen may contribute to the neonate's predisposition to T-cell mediated infection.