Most models proposed to explain the disease-associated expansion of (CTG)n.(CAG)n and (CGG)n.(CCG)n trinucleotide repeats include the formation of slipped strand DNA structures during replication; however, physical evidence for these alternative DNA secondary structures has not been reported. Using cloned fragments from the myotonic dystrophy (DM) and fragile X syndrome (FRAXA) loci containing normal, premutation, and full mutation lengths of repeats, we report the formation of novel alternative DNA secondary structures that map within the repeat tracts during reannealing of complementary strands, containing equal lengths of repeats, into linear duplex DNA molecules. Linear duplex DNA molecules containing these alternative DNA secondary structures are characterized by reduced electrophoretic mobilities in polyacrylamide gels. These alternative secondary structures are stable at physiological ionic strengths and to temperatures up to at least 55 degrees C. Following reduplexing, the CAG strand of the (CTG)n.(CAG)n repeats is preferentially sensitive to mung bean nuclease, suggesting the presence of single-stranded DNA in the alternative DNA structure. For (CTG)17, which is a repeat length found in normal individuals, less than 3% of the DNA molecules formed alternative DNA structures upon reduplexing. DNA molecules containing (CTG)50 or (CTG)255, which represent premutation and full mutation lengths of triplet repeats, respectively, formed a heterogeneous population of alternative DNA structures in >50% of DNA molecules. The complexity of the structures formed increased with the length of the triplet repeat. The relationship between repeat length and the propensity of formation and complexity of the novel structures correlates with the effect of repeat length on genetic instability in human diseases. These are the first results consistent with the existence of slipped strand DNA structures. The potential involvement of these structures, which we term S-DNA, in the gentic instability of triplet repeats is discussed.