The term X syndrome involves several dilemmas. The terminological dilemma is that this term is used to describe microvascular angina pectoris, as well as Reaven's metabolic-hormonal syndrome and our 5H syndrome [association of hyperinsulinism with arterial hypertension, hyperlipoproteinaemia, hyperglycaemia (NIDDM) and hirsutism]. It is probable that the coronary X syndrome is frequently conditioned by the hormonal-metabolic X syndrome. The pathogenetic dilemma is that it is not clear why in microvascular angina pectoris the coronary circulation does not possess an adequate reserve for vasodilatation during exercise or in response to some pharmacological stimuli. This could condition hyperinsulinism in hypertonic subjects with NIDDM with an early disorder of paracrine endothelial relaxation mechanisms (EDRF-NO), with a predominance of vasoconstrictor mechanisms (endothelin-1). In Reaven's syndrome X it is not clear whether insulin resistance of the postreceptor type is a primary inborn phenomenon which is compensated by insulin hypersecretion or whether it is a secondary phenomenon, which develops ex post and by which the target tissues defend themselves against an excessive effect of insulin (e.g. down regulation receptors) in primary disorders of its secretion.