Primary (essential) hypertension has recently been related to calcium deficiency, rather than excess. The evidence used to support this hypothesis includes surveys showing lesser dietary intake of calcium, lower levels of ionized calcium in the blood, and reduction of blood pressure with calcium supplements. This critique examines each of these points and the theoretical construct used to explain the hypothesis. We conclude that the theoretical construct is based on the use of only a portion of available experimental data and the clinical evidence remains inconclusive. Until the hypothesis is supported further, calcium deficiency should not be accepted as a mechanism responsible for hypertension and calcium supplements should be used with caution.