We set out to determine the prevalence of bacteriuria in elderly patients institutionalised in 10 homes in Northumberland. Single mid-stream urine (MSU) specimens were collected from 161 asymptomatic patients aged 64-101 years. Thirty-four (21%) samples had > 100 white blood cells (WBC/microliter) on microscopy as well as a significant bacteriuria of > 10(8) bacteria/l, being suggestive of a true urinary tract infection. A total of 115(71%) samples had < 100 WBC. Seventy-five (46%) samples could be described as 'contaminated' or 'infection unlikely' but of these only 31 (19.2% of all samples) were reported as 'contaminated' by the laboratory. This study confirms previous work in America to indicate the high level of bacteriuria in institutionalised elderly patients as well as the very high number (46%) who may show bacteriuria but no rise in the number of WBC suggesting contamination only. Nurses and GPs must be very cautious in interpreting MSU results from such patients.