To evaluate the value of beta-2-microglobulin as an indicator of acute and long-term cis-platinum-induced nephrotoxicity, 51Cr-EDTA clearance and serum concentration and urinary excretion of beta-2-microglobulin were measured in 18 patients treated with a regimen including cis-platinum. Before treatment all values were within the normal range. During treatment 51Cr-EDTA clearance decreased from 108 to 90 ml/min/1.73 m2 (P less than 0.02). The decrease was irreversible, while a transient 2 to 5-fold increase in beta-2-microglobulin excretion in the urine was seen during treatment. Serum beta-2-microglobulin remained unchanged. The decrease in 51Cr-EDTA clearance was not correlated to either the peak increase in the beta-2-microglobulin excretion or to the time of occurrence of the peak (R = 0.3). Thus, it is not possible to predict the long-term nephrotoxicity of cis-platinum by measuring the beta-2-microglobulin excretion during treatment.