Abstract ASTM Committee E-24 on Fracture has recently standardized the crack tip opening displacement (CTOD) measurement method. Three major differences between the ASTM E 1290-89 method and British Standard BS5762:1979 are outlined, and the influence of these differences on the critical crack initiation CTOD value is studied. The study is based on a typical fatigue crack and slow crack growth front profile. A difference of up to 6% in the CTOD values between the two standards, irrespective of the slow crack growth size, was found in the present study. However, a marginal ditterence of only 2% in the initiation CTOD value could be observed for the slow crack growth situation considered.