Heteroplasmy is the presence of more than one mtDNA sequence within an individual, which can lead to ambiguous results. The results from the current study found that the multiple sequences present in individual samples resulted from heteroplasmy, not from contamination. The study found that the frequency of heteroplasmy in human mtDNA differed among age groups; statistical analysis showed a significant increase of heteroplasmy with age. This indicates that the age of the individual in question should be considered when interpreting mtDNA typing results. This is particularly important in missing person cases in which a significantly older person is used as a reference for a missing younger maternal relative. The frequency of heteroplasmy in mtDNA also apparently differs across tissue types, appearing to be more frequent in muscle tissue. This suggests that tissue samples other than muscle should be typed, if possible, when conducting mtDNA analysis for forensic cases. This would minimize the potential interpretation issues associated with heteroplasmy. The SSO probe system consisted of 16 SSO probes that detect sequence polymorphisms within five regions of the mtDNA control region. Five regions of hypervariable region II (HVII) of the control region were studied in blood, muscle, heart, and brain tissue samples collected from 43 individuals during autopsy. An initial search for heteroplasmy was conducted with the SSO probe system. Five of the 43 individuals evidenced heteroplasmy in mtDNA. Samples in which multiple probe signals were detected within a region were sequenced for the HVII region so as to verify the typing-strip results. 6 tables, 3 figures, and 53 references