Tamm-Horsfall (TH) mucoprotein has been suggested to play a lithogenetic role in calcium-oxalate nephrolithiasis. However it is still debated whether it promotes or inhibits crystal growth and aggregation. To make clear the role played by this mucoprotein, we have carried out the following experiments: 1) the urinary excretion of TH has been evaluated by radial immunodiffusion in 27 recurrent idiopathic CaOx stone formers and in 35 controls; 2) in a metastable solution of CaOx the effect of TH addition on crystal growth has been monitored; 3) in whole urine the effect of TH addition on crystal aggregation has been assayed by an aggregometer. Urinary excretion of TH is significantly lower in stone formers. TH does not seem to promote crystal growth, while it is effective on crystal aggregation. These data seem to suggest that the reduced excretion of TH in nephrolithiasis may be a lithogenic risk factor.