Familial hypocalciuric hypercalcemia is said to be an extremely rare condition but is clinically important because it can be confused with primary hyperparathyroidism. The biochemical features of the two conditions are similar, but the former is benign while the latter can have serious clinical consequences with patients occasionally proceeding to parathyroidectomy. It is therefore important to differentiate accurately between the two. With this in mind it would be useful to know the prevalence of familial hypocalciuric hypercalcemia when considering the differential diagnosis of primary hyperparathyroidism. However, as far as we are aware, no estimate of the prevalence of this condition can be found in the literature. We describe how an estimate was made of the prevalence of familial hypocalciuric hypercalcemia in the west of Scotland. We estimate the prevalence to be 1 in 78,000 at least.