SUMMARYA series comprising 105 cases of hyperparathyroidism is presented. Ninety‐five of these cases were successfully parathyroidectomised and have been satisfactorily followed up.The principal aim of the investigation has been to establish the incidence of hypertension in hyperparathyroidism and to find out if any correlation exists between the renal damage and the occurrence of hypertension. The possibility of a relationship between other symptoms of hyperparathyroidism and hypertension is of interest, and the behaviour of the blood‐pressure and renal function following parathyroidectomy has been investigated.The following results were obtained:—1Seventy per cent. of the cases had hypertension some time during the observation period, the distribution being as follows:50 per cent. constant hypertension; 20 per cent. hypertension before, but normal blood‐pressure after, parathyroidectomy; and 30 per cent. developing hypertension after parathyroidectomy.2The presence of renal calculi or severe renal infection was not demonstrably correlated to hypertension. Nephrolithiasis seemed to be the predominating symptom in the normotensiv group. If any correlation between nephrocalcinosis and hypertension existed at all, it was a slight one. The patients with parathyroid osteitis were mainly referable to the hypertensive groups and were infrequent in the normotensive group and the group with post‐operatively normalised blood‐pressures.3The mortality was 20 per cent. The causes of death were azotæmia in eleven cases, sequek of hypertension (cerebral vascular accidents and cardiac failure) in eight, and other causes in two cases (suicide and arteriosclerosis).4A striking correlation was found between the extent of renal damage (reflected by functional studies and routine biopsies) and the severity of hypertension. The results also indicate that the impairment of renal function preceded the development of hypertension, which implies that the hypertension of hyperparathyroidism may be of renal origin.5The actual mechanism producing these manifestations is not clear from the findings in this investigation. Further studies are necessary, and are, in fact, proceeding in