Repeating DNA sequences, such as telomeres, centromeres, and micro- and mini-satellites, comprise 50% of the genome and play important roles in regulatory and pathogenic mechanisms. In order to study structures and functions of such repeating sequences, it is important to have simple and efficient methods for making them in vitro. Here, we describe the efficient and convenient expansion of repetitive telomeric and minisatellite DNA sequences starting from small synthetic templates to final product lengths of several hundreds to thousands of nucleotides by the thermostable DNA polymerase from Thermococcus litoralis (Vent DNA polymerase). This enzyme was so far unknown to catalyze repeat expansion. Either single-stranded or double-stranded DNAs could be produced, depending on nucleotides present. Compared to earlier results obtained with other enzymes, the expansion reaction is highly efficient both in its yield and product length, and proceeds without thermal cycling. Moreover, the products are characterized by a narrow length distribution.