The mitochondrial haplogroup T, characterized by the nucleotide motif 16126C-16294T in the hypervariable segment I (HVS I), is one of the most frequent among Europeans. It has been shown that this haplogroup includes the only well-resolved subgroup, T1, but that other HVS I sequences cannot be differentiated into subgroups due to possible homoplasies at nucleotide positions 16292, 16296 and 16304, leading to the reticulations in the topology of phylogenetic networks. To study the problem of molecular instability at these positions, we have performed an analysis of 159 previously published West Eurasian HVS I sequences belonging to haplogroup T, together with 12 new HVS I sequences of Eastern Slavs. These 12 sequences represent 16.9% of a total of 71 samples analysed and identified as haplogroup T mtDNAs by RFLP analysis in this study. A search for rare point mutations associated with different combinations of nucleotides 16292T, 16296T and 16304C within the haplogroup T sequences, and specific to certain populations or a group of closely related-by-descent populations, was performed. This analysis revealed 11 marker mutations, each of which was characteristic for a certain group of linguistically or geographically close individuals - the Adygei, Germans, Kazakhs and linguistic isolates of the Eastern Italian Alps. The occurrence of these rare population-specific polymorphisms in association with various combinations of mutations at positions 16292 and 16296 on the haplogroup T background provides evidence of molecular instability at these nucleotide positions. Molecular instability in the haplogroup T HVS I sequences is also suggested by multiple independent losses of the haplogroup T diagnostic nucleotide variants in different populations. The results of the present study suggest that identical haplogroup T HVS I sequence types might have arisen independently in different human populations.