OBJECTIVE The current standard for gastric emptying studies are radioactive isotope methods. [13C]breath tests have been developed as a nonradioactive alternative. The aim of this study was to validate a [13C]acetate breath test as a measure of gastric emptying of the liquid phase both in liquid and semisolid test meals by simultaneous radioscintigraphy. METHODS Thirty-five patients with dyspeptic symptoms and 20 healthy volunteers were tested using a semisolid oatmeal or a liquid test meal. Both test meals were labeled by 150 mg sodium [13C]acetate and (in patients) by 45 MBq 99mTc-albumin colloid. Half-time of gastric emptying was calculated after curve fitting of the 13C exhalation to a modified power exponential function. 99mTc-albumin emptying was measured by conventional radioscintigraphy. RESULTS The half-emptying times for the [13C]acetate breath test closely correlated to those measured by radioscintigraphy both for semisolids (r = 0.87) and liquids (r = 0.95). The time of maximum 13CO2 exhalation was itself a reliable parameter compared with the half-emptying times obtained by scintigraphy (r = 0.85 for semisolids; r = 0.94 for liquids). CONCLUSIONS The [13C]acetate breath test is a reliable and noninvasive tool for the analysis of gastric emptying rates of liquid phases without radiation exposure.