The length of time a named antibiotic should be given to eradicate a urinary tract infection in the adult non-pregnant female seen in a general practitioner's surgery has been further investigated. A three-day course of amoxycillin is superior to a one-day course of the same drug in the same dose, in sterilizing the urine and in providing relief of symptoms. A previous study in the same population employing the same criteria but comparing a ten-day course with a three-day course (Charlton et al., 1976), showed no difference in the cure rate bacteriologically and symptomatically. We therefore recommend that a three-day course of antibiotic is the preferred length of time for the treatment of this group of patients.