Forty patients suffering from symptomatic urinary tract infection were treated with pivmecillinam, the pivalolyoxymethyl ester of mecillinam, a new β-lactam antibiotic. Twenty-six of the 40 patients responded satisfactorily to treatment. Relief of symptoms and sterilization of urine was achieved in 87% of patients with uncomplicated infections but in only 35% of those with complicated disease. The bactericidal activity of the patient's urine during therapy against their infecting organism was studied and compared to standard disc sensitivity testing as a means of predicting the outcome of therapy. The bactericidal test proved to be superior in complicated urinary tract infections.