An accurate and reproducible measurement method for joint motion is essential for classification of success or failure in therapeutic intervention. Digital goniometry is increasingly used as a method of classification for knee joint excursion. The reliability of goniometry however remains debatable. Aim of the study was to determine both intra- and inter-rater reproducibility in degrees, with an electronic digital inclinometer (EDI 320) for active and passive maximum flexion and active maximum extension of the knee joint and to determine the reproducibility of active and passive range of motion. A classical crossover design, with strict measurement protocol was used. Two raters measured 72 knee motions each, in 42 healthy subjects in four sessions. The smallest detectable difference (SDD) was calculated by using adjusted Bland and Altman plots for each knee excursion. No differences in joint excursions between the sexes were found. Passive maximum flexion showed larger excursions than active maximum flexion with additional higher levels of reproducibility. SDDs for inter-rater comparisons yielded: 0+/-3.9 degrees for active maximum extension, 0+/-7.4 degrees for active maximum flexion, 0+/-6.4 degrees for passive maximum flexion, 0+/-7.6 degrees for AROM and 0+/-5.4 degrees for PROM. Intra-rater SDDs showed increased reproducibility by 0.4-1.9 degrees. We conclude that interpretation of knee joint excursions in clinical settings is with these SDDs. Clinical and statistical differences in research settings within these SDDs are not a true difference but should be attributed to measurement error.