The results show that Y-STRs can identify nonpaternity cases with high accuracy, but must be validated with autosomal STR (A-STR) in order to establish paternity. This proposed two-stage approach is less costly than the standard approach and requires typing only of the alleged father and the son. This is particularly useful when the mother is not available for testing. The study randomly selected DNA samples of 22 alleged father/son/mother trios from the database of tested cases in the laboratory services of the Hospital Centre CHU Hedi Chaker of Sfax in Tunisia. The 22 pairs were genotyped for a set of Y-STR and A-STR markers. The mothers were also genotyped for A-STRs. For A-STR, PCR amplification was performed in Gene Amp PCR system 9600, using the AmpFISTR-SGM Plus PCR Amplification Kit. For Y-STR, 12 Plex-amplification was performed by the commercial kit Y-Plex 12. This paper also describes the statistical methods used. 1 table, 1 figure, and 34 references