Labile blood pressure elevation is believed to have less clinical significance than "fixed hypertension." This assertion was examined in the Framingham cohort of 5209 men and women followed for 20 years for the development of cardiovascular events in relation to three routinely measured blood pressures at each of 10 biennial examinations. Variability of pressure judged from the standard deviation about the mean of three pressures was not a consistent characteristic of subjects from one examination to the next (r = 0.07). Higher pressures were more labile than low ones, so that "fixed hypertensives" actually had more labile pressures than did so-called labile hypertensives. Lability, also increased with age. Labile hypertension, determined during a 1-hour period of observation, adds nothing to the ability of the mean blood pressure to predict cardiovascular disease. The mean, minimum and maximum of three pressures measured during an examination were equally efficient predictors of cardiovascular disease. In multivariate analysis, for any given average pressure, risk of cardiovascular events was unaffected by the degree of variability of the pressure. It is recommended that the average of a series of pressures be used to determine risk, preferably over more than one examination.