It has recently been suggested that a combination of C-Reactive Protein (CRP) and beta-2-microglobulin (beta 2M) can be used to devise a simple prognostic model for patients with multiple myeloma. In this study we have measured serum beta 2M, CRP and thymidine kinase (STK) in a series of 215 samples to determine their value as a monitor of disease status. A longitudinal study was also performed with 6 individual patients. CRP levels did not correlate with disease status. The mean of the stable (23.62 mg/L) and the progressive disease 23.64 mg/L) groups were almost identical (t = 0.003; p = NS) with ranges of < 5-150 mg/L and < 5-100 mg/L respectively. There was no correlation between CRP and STK (r = 0.11) or CRP and beta 2M (r = 0.05). In longitudinal studies, CRP did not necessarily reflect changes in disease activity. We conclude that CRP measurements are not valuable as a monitor of disease activity in patients with myeloma.