BACKGROUND: The current investigation examined the inter- and intra-tester reliability of knee joint angle measurements using a flexible Penny and Giles Biometric electrogoniometer. The clinical utility of electrogoniometry was also addressed. METHODS: The first study examined the inter- and intra-tester reliability of measurements of knee joint angles in supine, sitting and standing in 35 healthy adults. The second study evaluated inter-tester and intra-tester reliability of knee joint angle measurements in standing and after walking 10 metres in 20 healthy adults, using an enhanced measurement protocol with a more detailed electrogoniometer attachment procedure. Both inter-tester reliability studies involved two testers. RESULTS: In the first study, inter-tester reliability (ICC[2,10]) ranged from 0.58-0.71 in supine, 0.68-0.79 in sitting and 0.57-0.80 in standing. The standard error of measurement between testers was less than 3.55 degrees and the limits of agreement ranged from -12.51 degrees to 12.21 degrees . Reliability coefficients for intra-tester reliability (ICC[3,10]) ranged from 0.75-0.76 in supine, 0.86-0.87 in sitting and 0.87-0.88 in standing. The standard error of measurement for repeated measures by the same tester was less than 1.7 degrees and the limits of agreement ranged from -8.13 degrees to 7.90 degrees . The second study showed that using a more detailed electrogoniometer attachment protocol reduced the error of measurement between testers to 0.5 degrees . CONCLUSION: Using a standardised protocol, reliable measures of knee joint angles can be gained in standing, supine and sitting by using a flexible goniometer.