Direct sequencing of PCR products using Taq DNA polymerase with dye-labeled dideoxy chain terminators results in traces with uneven peaks. The peak height variations reflect the disproportionate rate of incorporation of deoxynucleotides vs. their analogs, a phenomenon that is highly dependent on the neighboring DNA sequence. Such peak height variations make it difficult to call bases correctly or to interpret whether or not a polymorphism is present. We have systematically examined pairs of sequence-tagged sites that vary at only one nucleotide to determine how a single base change will affect the peak heights of neighboring bases. We have found that the peak height of a particular base can often be predicted from the knowledge of just one or two nucleotides 5'- to the base in question. We have also observed several artifacts that occur consistently in the sequencing traces. These artifacts can be misinterpreted as polymorphisms or can obscure the real peak at that site. The empirically derived trends presented in this report can be utilized profitably when one is editing sequence data or examining them for polymorphisms and mutations.