Fifty-five consecutive admissions to an acute geriatric unit were studied prospectively. Individuals were classified according to the obvious presence or absence of an active disease process on admission and their serum C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) were measured then and five days later. There was no significant difference between the ESR values in the two groups either on admission or at day 5, nor was there any significant change between admission and day 5. In contrast, CRP values were very significantly higher in the active than in the non-active group and there were marked changes over the 5-day period corresponding to changes in clinical condition. These results indicate that the serum CRP concentration is superior to the ESR as an objective, non-specific marker for disease activity in the elderly.