Multiaxial fatigue behaviour of box-welded (wrap-around) joints in a JIS SM400B steel (12-mm-thick plate) was examined using a biaxial fatigue test facility. For the specimen, two stiffeners were attached to a main plate by a CO2 semi-automatic welding procedure. Residual stress measurements and finite element (FE) analyses were also performed. Fatigue tests were performed under both uniaxial and biaxial (mainly out-of-phase) cyclic loads, and both results were compared and examined. It was found that fatigue cracks in the biaxial fatigue test specimens were initiated at the boxing-weld toes and propagated almost in the direction of the lateral loads. This is considered to be due to the dominant direction of tensile residual stresses from welding and the stress concentration in the vicinity of the boxing-weld toe. From the relation between the strain range near a weld toe, Δε5, and the fatigue lives, it was found that crack initiation life, Nc, was almost equivalent in the biaxial and uniaxial fatigue tests, while the failure life, Nf, was slightly longer in the biaxial tests. However, when the fatigue lives are put in order using the stress range near a weld toe, Δσ5, the crack initiation life, Nc, in the out-of-phase biaxial tests (phase difference of π) is ∼30% lower than in the in-phase biaxial and uniaxial tests, while the failure life, Nf, was almost equivalent in the biaxial and uniaxial tests. From these results, it is concluded that an increase in Δσ5 (lowering of the minimum value of σ5), induced by the out-of-phase lateral loads, leads to an increase in fatigue damage where the high tensile welding residual stresses exist in the vicinity of the boxing-weld toe. Finally, a simple life estimation for the biaxial fatigue tests was made using FE analyses and the results of the uniaxial fatigue tests, proving that the effects of the lateral loads should be taken into consideration. © 1999 Blackwell Science Ltd.