Purpose: Air plethysmography (APG) has the potential to help evaluate different treatments for the prevention of recurrence of venous ulcers; however, there are little reported data on the variation and reliability of the different parameters. This study aimed to assess the variation in different APG parameters in patients with chronic venous disease and to evaluate the reliability of APG in test-retest situations.Method: Seventeen patients (18 limbs) with chronic venous disease were recruited into this study. Subjects were asked to undergo tests on two occasions, 1 to 6 weeks apart. Three tests were performed at each visit, and three patients had 10 tests performed at one visit. The coefficients of variation were calculated for repeated measurements and test-retest reliability, and the differences between the means of three tests and the 10 tests were also analyzed.Results: The coefficients of variation for the repeated measurements ranged from 7.5% to 27% for the majority of parameters of APG. The differences between the means of three tests and the means of 10 tests were less than 10% in this study. The coefficients of variation of method error were approximately 10% in test-retest measures.Conclusions: This study has shown that evaluations of calf pump function and venous reflux using APG display variations in repeated measurements and in the test-retest measures. The variations found within patients and on retesting patients on different days suggest that APG is very unlikely to be able to detect small changes in the parameters of venous reflux and calf pump function. It is essential to understand the inherent variation of APG measurements when they are used to assess treatments that are designed to improve venous function.