The Lifson‐Roig theory for the helix‐random coil transition in polyglycine has been modified to take side‐chain interactions into account. The modified theory is developed specifically for poly‐L‐alanine which probably contains a β1‐α4 hydrophobic bond. A conditional probability, expressing the strength of such a hydrophobic bond, is evaluated from the Nemethy‐Scheraga data for the free energy of formation of the bond. This, together with the conditional probabilities for the states of the backbone chain, permit an evaluation of the partition function and properties of the helix‐random coil transition. It is seen that the hydrophobic bond renders the poly‐L‐alanine helix much more stable than the corresponding polyglycine one, in agreement with data obtained by Gratzer and Doty.