Abstract We recently published a theory dealing with the support of a quiescent prominence comprised of a number of adjacent magnetoplasma loops, the principal feature of which was a balance between the upward force 1 2 L ′ I 2 due to an electric current I flowing around a typical prominence loop of vertical induction gradient L ′ and the downward force M g of gravity, where M is the mass of the upper section of the loop. In this Letter we deal with the question of the stability of prominences; we show that they are unstable above a critical height h c ≈ 1.7 ℘ , where ℘ is the pressure scale height in the corona. For typical prominences ℘ ∼ 36 000 km , which gives h c ∼ 61 000 km . We have also determined the upward speed of a prominence in the unstable regime and find that this increases to about 600 km s −1 at a distance of two solar radii.