Annually, members of the English Speaking Working Group of the International Society for Forensic Genetics are invited to participate in a Relationship Testing Workshop. In 2015, 64 laboratories participated. Here, we present the results from the 2015 workshop, which included relationship testing of blood samples from a mother, two children and an alleged father. Furthermore, the laboratories filled in a questionnaire concerning the laboratory strategies and routines. Finally, the laboratories were encouraged to do a paper challenge concerning the relationship between a child (boy) and an alleged grandfather with statistical calculations including results of autosomal STRs and Y-chromosomal STRs. The paper challenge showed large variations in the biostatistical calculations in cases of genetic inconsistencies observed in Y-chromosomal STR typing results.