The epidemiology, pathogenesis, significance and management of hypertension in diabetic subjects are discussed. In Type 1 diabetes the presence of diastolic hypertension is closely related to the presence of diabetic nephropathy, from the stage of persistent proteinuria onwards. There may also be some elevation of systolic pressure. The apparent increased prevalence of hypertension in Type 2 diabetes is largely explicable, directly or indirectly, by obesity but there may be an excess of systolic hypertension among elderly patients. Hypertension in the diabetic population is associated with an increased incidence of both microvascular and macrovascular complications, but whether the high blood pressure is causal is not clear. The possible roles of sodium and insulin, the renin-angiotensin system, catecholamines and physical factors are explored. All current antihypertensive agents have additional limitations and disadvantages when used in diabetic patients: diuretics and beta-blockers are probably the initial drugs of choice. Only in the case of diabetic nephropathy is there yet reasonable evidence of antihypertensive treatment reducing the rate of progression of the disease.