The aim of this investigation was to develop a method for the prediction of the high-strain multiaxial fatigue life (low-cycle fatigue) of a steel, based on high-strain uniaxial fatigue data. In this study, thinwalled cylindrical specimens were subjected to fully reversed high biaxial strains using differential pressure and axial load. The directions of the principal strains were maintained parallel and perpendicular to the specimen axes. Five different strain ratios were investigated covering both the inphase and fully out-of-phase conditions. The material selected for this investigation was a medium carbon steel; C-1018. It was found that a maximum principal strain theory, based on the plastic components of the applied strains, taking into account the strain in the thickness direction, resulted in a correlation between the biaxial and uniaxial results which was either good or conservative. Under inphase conditions the low-cycle fatigue life could be predicted using Nf = Nf0 a(ε1+ε2) and under fully out-of-phase conditions using Nf = Nf0 aε1 where ε1 and ε2 are the ordered applied plastic strains and the constants “Nf0” and “a” are obtained from uniaxial high-strain fatigue tests.