The relationship between urinary metallothionein and kidney and liver cadmium levels was examined in 68 active and retired smelter workers. Metallothionein was analyzed by a radioimmunoassay and liver and kidney cadmium levels were determined by in vivo neutron activation. Four workers suffered from severe renal dysfunction and excreted high amounts of total protein and beta 2-microglobulin and greater than 1 mg metallothionein/g creatinine. In the remaining 64 workers the urinary metallothionein levels correlated significantly with the cadmium levels in both liver and kidney. Similarly, in these individuals urinary metallothionein was significantly related to cadmium in blood and urine. These results demonstrate that urinary metallothionein is a sensitive biological indicator of cadmium exposure and body burden, before the onset of severe renal dysfunction.