In patients with light-chain myeloma or primary AL-amyloidosis, 24-hr light-chain excretion in the urine is considered an essential marker of the tumor mass. However, 24-hr urine collection and analysis may be cumbersome and prone to inaccuracy. Recently, a sensitive immunonephelometric assay for immunoglobulin free light chains (FLC) in the serum was developed. We sought to determine whether the serum level of monoclonal FLC could be used as an indicator of urinary excretion and disease evolution. Seven patients with light-chain myeloma and AL-amyloidosis were studied, all of which had a monoclonal FLC that could be detected in the urine using standard methods. In four of these patients, follow-up revealed a remarkable correlation between FLC serum levels and daily urinary excretions. The ratio of serum level to urinary light-chain excretion, although stable in a given patient, was extremely variable between patients. In the three remaining cases featuring hardly measurable amounts of light chain in the urine, the serum FLC assay proved sensitive enough for correlation with clinical events. Thus, immunonephelometric measurement of serum FLCs is a reliable method for the follow-up of patients with light-chain secreting monoclonal gammopathies.