To assess the reliability and validity of real-time ultrasonic estimation of bladder volume we conducted an overview of the published literature identified using MEDLINE search (1966-96) and scanning of the bibliographies of known primary and review articles. Short-listed papers were classified into reliability (observer agreement) and validity (comparison of ultrasound estimation with actual bladder volume) studies. Study selection and data extraction were performed independently in duplicate. There were 81 subjects enrolled in 3 reliability studies and 504 subjects in 16 validity studies. Where reported, the index of concordance for reliability ranged from 0.923 to 1.00, while for validity it ranged from 0.914 to 0.983. However, there were several inadequacies in the design, conduct and analysis of these studies, leaving some doubt about the trustworthiness of the high levels of reliability and validity reported in the literature.