A general theory of nontenuous cold relativistic helically symmetric steady flows is developed. Both self‐fields and radial effects are included. The fluid and the Maxwell equations are reduced to a system of equations for three scalar functions: a stream function, a flux function, and an electrostatic potential. These equations are expanded under the assumption of small perpendicular momenta and a simplified set of ordinary differential equations is obtained. Two cases are studied in detail. The first case is of a system externally driven by a magnetic wiggler. The second is the self‐excited system, with no externally applied wiggler. The equilibria described here are of a practical importance for free‐electron lasers which employ high‐density thick beams.