Over six years (1978-83, inclusive) weekly laboratory records of organisms causing urinary tract infection in women aged 15-25 not attending hospital were kept prospectively and analysed. The incidence of infection with Staphylococcus saprophyticus defined by age and sex was confirmed. This organism caused an increasing proportion of infections in young women over the six years studied, and these infections showed noticeable seasonality. All but four isolates of S saprophyticus were sensitive to all the commonly used antimicrobial agents that were tested. This might be because the organism is not often present in the body as a commensal and therefore not subject to the selection pressures exerted by such agents. As infection with S saprophyticus has different clinical connotations from infection with other coagulase negative staphylococci it should be differentiated from them in routine laboratory practice.