Plasma viscosity (PV) has replaced the erythrocyte sedimentation rate (ESR) as a routine laboratory test in many hospitals. The finding of a normal PV but raised ESR in a case of biopsy proved giant cell arteritis (GCA) cast doubt on this substitution in cases of suspected GCA. To assess the equivalence of PV and ESR in the diagnosis of this disease 40 suspected cases were prospectively investigated with both tests. The correlation between the two tests was good (r = 0.742, p less than 0.0001). The substitution of one test for the other would appear to be justified in most cases of suspected GCA. In the presence of biopsy proved disease, however, the PV and ESR each produced 13.3% false negatives. These occurred both in combination with and independently of the other test showing that, when in error, the two tests may not be equivalent. In cases of doubt the performing of both PV and ESR tests together improves but does not achieve complete diagnostic accuracy. Clinical judgment based on careful assessment of all available symptoms and signs must remain the foundation of diagnosis.