The experience of our self-help organisation shows that the reason patients with symptoms of magnesium (Mg) deficiency do not get Mg therapy is acceptance of an inappropriate lower limit of the reference values for serum Mg concentration. The commonly designated low limit of the normal range seems to have been selected from values obtained for symptomatic patients. It is below levels that exist in patients with marginal deficiencies that can predispose to development of pathologic findings, so that the prevalence and importance of this disease is insufficiently considered. The lower reference limit of the normal population is erroneously regarded as a diagnostic criterion that excludes Mg deficiency when the serum level is even slightly above the reference limit that only excludes normality at lower levels. It is a statistical error to use the confidence limits of the normal population as the exclusion limit for those with abnormal Mg status.