We present the results of the 2013 Relationship Testing Workshop of the English Speaking Working Group of the International Society for Forensic Genetics. A total of 56 laboratories participated. The exercise included relationship testing of blood samples from an alleged mother and two children. Furthermore, the laboratories filled in a questionnaire concerning the laboratory strategies and routines. Finally, the laboratories were encouraged to do a paper challenge with statistical calculations including autosomal STRs and X-chromosomal STRs. In the relationship testing exercise, the two children were truly a biological child and a biological grandchild of the alleged mother. The exercise revealed that the use of X-STRs was informative and excluded the alleged mother as the mother of her grandchild. The paper challenge showed large variations in the biostatistical calculations in cases of rare events such as possible silent alleles, genetic inconsistencies and in calculations involving X-chromosomal STR genotyping results.