OBJECTIVES: To determine prevalence and demographics of two myosin-binding protein C (MYBPC3) mutations that affect ragdolls (R820W) and Maine coons (A31P) in the British Isles. METHODS: From the database of a genetic testing laboratory samples from 2018 ragdolls and 742 Maine coons were analysed with respect to mutation status, age, sex and county of origin. The actual prevalence was compared to the expected Hardy-Weinberg prevalence by chi-squared test. RESULTS: The prevalence of the R820W mutation in ragdolls was 27% (25·6% heterozygous, 1·4% homozygous), and that of the A31P mutation in Maine coons was 39·4% (36·4% homozygous, 3% heterozygous). There were more female cats (69·5% ragdoll, 70·3% Maine coon). The median age was 6·4months (ragdolls) and 5·9 months (Maine coons). Cats from more than 60 counties were represented for each breed. The difference between the expected and observed allele frequency was significant in Maine coons (P=0·047) but not in ragdolls (P=0·092). CLINICAL SIGNIFICANCE: This is the first report of prevalence and demographics of the R820W and A31P mutations in ragdolls and Maine coons, respectively, in the British Isles. The prevalence is high, which is of relevance for breeding and screening programmes. The significant difference in genetic distribution may suggest early death of homozygous Maine coons.