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Introduction: Of newly diagnosed cases of haemophilia B, the proportion of sporadic cases is usually 50% of severe cases and 25% of moderate/mild cases. However, cases presumed to be sporadic due to family history may not always be sporadic. Few case reports have been published on mosaicism in haemophilia B. Aim: The present study aimed to trace the origin of the pathogenic variant in a well‐defined cohort of sporadic cases of haemophilia B by haplotyping markers. It also aimed to determine the frequency of mosaicism in presumed non‐carrier mothers. Methods: The study group was 40 families, each with a sporadic case of haemophilia B analysed in two‐to‐three generations by Sanger sequencing, haplotyping and using the sensitive droplet digital polymerase chain reaction (ddPCR) technique. Results: In 31/40 (78%) of the families, the mother carried the same pathogenic variant as her son, while Sanger sequencing showed that 9/40 (22%) of the mothers did not carry this variant. Of these variants, 2/9 (22%) were shown to be mosaics by using the ddPCR technique. 16/21 carrier mothers, with samples from three generations available, had a de novo pathogenic variant of which 14 derived from the healthy maternal grandfather. Conclusion: The origin of the pathogenic variant in sporadic cases of haemophilia B is most often found in the X‐chromosome derived from the maternal grandfather or, less often, from the maternal grandmother. Mosaic females seem to be found at the same frequency as in haemophilia A but at a lower percentage of the pathogenic variant. [ABSTRACT FROM AUTHOR]