The following parameters were considered in studying 43 subjects under treatment for urinary infections with different degrees of renal insufficiency: the maximum blood concentrations, the plasmatic half-life, and the urinary concentrations of fosfomycin as a function of the plasma creatinine of the subject treated. The curve of the average maximum blood concentrations show a parabola-like increase, the plasmatic half-life followed the same tendency, while the curve of the average urinary concentrations presented an exponential decrease. The urinary concentrations were, nevertheless, always higher than 100 μg/ml regardless of the degree of renal insufficiency and even in the subjects who had a very high plasma creatinine. Given its absence of renal and systemic toxicity, the daily dosage of fosfomycin could remain unchanged regardless of the degree of insuffiziency