1. Human leucocyte AB antigens were determined by means of a lymphocyte toxicity test in 84 patients with essential hypertension and in 1000 blood donors.
2. The prevalence of HLA B8 was 16.4% in hypertensive patients and 8.9% in controls (P = 0.07).
3. The prevalence of HLA B12 was 34.5% in hypertensive patients and 26.9% in the control group (N.S.). In WHO stage III hypertension HLA B12 was found in six out of 10 patients.
4. The prevalence of HLA B15 was 1.2% in hypertensive patients and 6.4% in controls (P < 0.05).
5. In view of a previous report of HLA antigens in a Spanish diabetic population, this study does not support the suggestion of a genetic and possibly HLA-linked connection between essential hypertension and diabetes mellitus among the Spanish population.
6. A positive family history of hypertension tended to be more common in those patients with essential hypertension associated with HLA B8.