We investigated the impact of doping group IIIA elements (Al, Ga, In and Tl) on the electronic structure and stability of PbTe by first principles calculations. The impurity-induced defect level changes as a function of the charge state of the impurity. We find that Al and In prefer to act as donors while Ga and Tl tend to act as acceptors in PbTe. Our analysis supports the ‘impurity level’ model where an impurity-induced localized state overlaps either the conduction band or valence band of PbTe, but our results do not agree with ‘mix-valence’ (i.e. 2In2+ → In+ + In3+) or ‘auto-compensation’ (i.e. 2In0 → In+ + In−) models. Our calculations suggest that Tl and In are suitable dopants for improving the thermoelectric efficiency through enhancing the Seebeck coefficient for p- and n-type PbTe, respectively.