Recent experimental data suggest that the amount of 3(10) helical conformation in peptides and proteins might be larger than previously expected (Millhauser, G. L. Biochemistry 1995, 34, 3873-3877). This led us to explore a principal assumption of Zimm-Bragg theory of the helix-coil transition, that only one helical state can occur in polypeptides. In the present work we modify Zimm-Bragg theory to include the 3(10) helix as a competing helical state. Incorporation of the second helical state does not significantly change the nature of the helix-coil transition, preserving good agreement between theory and the large amount of relevant experimental data. The analysis of the model indicates that 3(10) helices should be on average shorter than alpha-helices. Also shorter polypeptides are predicted to have a significant ratio of 3(10) helical to alpha-helical hydrogen bonds. Moreover, as the total number of hydrogen bonds in the polypeptide decreases, the probability for a particular hydrogen bond to be in the 3(10) state rather than the alpha-helical state increases. The present analysis provides somewhat unexpected support for the recent proposal of the 3(10) helix as a thermodynamic intermediate in alpha-helix folding.