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Marfan syndrome is an autosomal dominant multisystem disorder that has an estimated incidence of 1 in 5000. It is caused by mutations in the FBN1 gene, which encodes the extracellular matrix protein type 1 fibrillin. Familial hypocalciuric hypercalcaemia (FHH), also inherited in an autosomal dominant pattern, is a rare benign disorder characterised by hypercalcaemia, hypocalciuria and relative hyperparathyroidism with normal or high plasma PTH levels, with an estimated incidence of between 1 in 10 000 to 1 in 100 000. We report a unique case of a 26-year-old man referred for investigation of hypercalcaemia, who also had clinical features of Marfan syndrome but no previous genetic investigations. Calculated fractional urinary excretion of calcium was low (0.0005) following correction of vitamin D deficiency, raising the possibility of FHH. Genetic testing for Marfan syndrome and FHH, via a hyperparathyroidism multiplex gene panel test, revealed a novel truncating variant in the FBN1 gene (c.8481T>G; p.(Tyr2827Ter)), consistent with Marfan syndrome; and a pathogenic truncating variant in the CaSR gene (c.741dupT; p.[Asp248Ter]), which confirmed the diagnosis of FHH. The patient’s mother was subsequently found to have mild hypercalcaemia (adjusted calcium 2.76 mmol/L) and is also heterozygous for the same CaSR mutation. Genetic testing of his father confirmed the presence of the same FBN1 gene mutation. This case illustrates the importance of making robust diagnoses in the era of modern genomic medicine, confirming FHH as the cause of hypercalcaemia means that no treatment is warranted and the patient can be reassured.