Immunofluorescence (IF) of urine sediment with antisera to light-chain immunoglobulins (LC) was used for patients with renal disease due to various monoclonal gammopathies. Urinary casts and/or masses from 20 of 27 patients (74%) of this group stained predominantly for one or another of the two LC, but from none of the 25 controls, who all had renal diseases other than monoclonal gammopathies. In all but one patient with monoclonal gammopathy the more intensely staining LC in the urine sediment was the same as that found in the paraprotein. For 68.7% of patients, the prevailing LC in urinary casts was the same as that prevailing in the casts of the renal biopsies. The sensitivity of IF of urine sediment for identifying patients with plasma cell dyscrasias was 74%, the specificity 100%, and the accuracy 86.5%. The positive predictive value was 100% and the negative predictive value was 78%. Globally, urine sediment IF was more sensitive than serum or urine electrophoresis (74% v 63% and 66.6%), but less than serum or urine immunoelectrophoresis (74% v 85%). In light-chain deposition disease, a disorder in which there are often no definite symptoms, urine sediment IF was more sensitive than both traditional methods. We conclude that IF of urine sediment is a useful test to identify patients with renal disease associated with monoclonal gammopathies.