The present disclosure generally relates to semiconductor structures and, more particularly, to smooth sidewall structures and methods of manufacture. The method includes: forming a plurality of mandrel structures; forming a first spacer material on each of the plurality of mandrel structures; forming a second spacer material over the first spacer material; and removing the first spacer material and the plurality of mandrel structures to form a sidewall structure having a sidewall smoothness greater than the plurality of mandrel structures.