This study was performed to assess: (1) the inter- and intrasubject variability of gastric emptying measurements by a scintigraphic method in 12 healthy subjects and 14 diabetic patients, and (2) the reproducibility of diagnosis of either the presence or absence of gastroparesis. To address this issue, radiolabeled solid-liquid meals were ingested by all subjects on two separate days. High intersubject variability of gastric emptying measurements was shown in both populations. Intrasubject variability was higher in diabetic patients than in healthy subjects. However, there was no significant difference between the means of any parameters obtained on two separate days. The reproducibility of the diagnosis of gastroparesis was excellent for all parameters; the solid half emptying time was the most reproducible parameter (92% in healthy subjects and 93% in diabetic patients). The means +/- SD of the difference between the two separate days' half emptying time results were -10.6 +/- 41.3 min for solids and -4.8 +/- 36.6 min for liquids and were not statistically different from zero. Only one difference between the half emptying time results on two separate days was not in the 95% confidence interval for both solids and liquids; however, this result came from a diabetic patient with obvious gastroparesis on both days. Thus: (1) in spite of high inter- and intrasubject variability, the scintigraphic method of measuring gastric emptying is highly reproducible for the diagnosis of gastroparesis; (2) reproducibility is better in healthy subjects than in diabetic patients; and (3) the knowledge of intrasubject variability allows assessment of the required sample sizes for pharmacological studies using prokinetic medications.