Purpose: Smartphones have become a staple in our society. They are essential not only for our day-to-day lives but also as a healthcare tool in the face of the emergence of a myriad of health-based applications. Analyze the reliability and concurrent validity of an iPhone app as compared with traditional goniometry.Methods: 21 subjects of our work centre, in 2016 had 4 measurements of their degree of knee flexion for a randomly chosen knee position: 2 by universal goniometry (UG) and 2 by the Goniometer-Pro (G-Pro) application. A different evaluator made each measurement. The radiographs were used as a gold standard to measure the actual knee flexion angle.Results: The difference between mean intra-group values was 3,148º (±2,669º) and 2,476º (±2,638º) for G-Pro. The difference between the mean intra-group values was 5.45º. Inter-observer consistency for UG was 0,990 and 0,993 for GPro; As regards validity, the values obtained were 0,976 for UG and 0,992 for G-Pro.Conclusions: The Goniometer-Pro app seems to be a reliable and accurate tool for determining the values of knee flexion. The values obtained are slightly more accurate than those of traditional goniometry.Implication: This study intend to promote the use of apps of systemic form in the health system, as a component integrated into the management of the health. This work not only compares the reliability of two methods of measurement, compare the reliability or validity with a gold standard, as it is the radiological measurement.