Creep rupture tests have been carried out on 1 Cr 1/2 Mo and 1/2Cr1/2Mo1/4V steels in the new (normalized and tempered) and in service exposed conditions. The results showed that the new materials were significantly stronger than the service exposed materials in tests of short duration, owing to the transient strengthening effects of carbon redistribution and carbide precipitation. With increasing test time the stress rupture strengths of the two conditions converged, as a stable microstructure was established in the new materials. The extrapolation of the new material data can lead to an underestimation of the long- term strength, continuing the steep/decrease in strength for the new material beyond the region in which experimental data are available. It is suggested that the stress rupture curve will in reality have a sigmoidal shape, as the results for new and service exposed materials must eventually come together.For the 1Cr1/2Mo steel, the creep damage developed in the longest tests depended on the condition. New material exhibited creep pore formation after relatively small creep strains of around 1% whereas the service exposed material exhibited no cavitation at creep strains of 5% after 10000 h testing. It was not possible in laboratory creep tests of 10 000 to 20 000 h duration to reproduce the creep damage typically found in components of this steel after prolonged service.