Serum beta-2-microglobulin (β2M) has been suggested as the most powerful prognostic factor in multiple myeloma (MM). This paper investigates its ability to detect remission and relapse in individual patients. A correlation analysis was carried out between β2M and M component determinations, at diagnosis and monthly during follow-up in 21 consecutive MM patients with normal renal function. A statistically significant correlation was observed in 52.4% only. The lack of correlation in the remaining cases was due to low β2M production at diagnosis, or independent fluctuation of these 2 parameters. Serum β2M proved a much less reliable parameter for the detection of tumour variations than simpler M-component determination.