An inertial sensor based gait analysis system is presented. Standard gait parameters, feet and hip ranges of motion (ROMs) and hip peak velocities, in medio-lateral and anterior-posterior directions, are validated in different experiments, including footswitches, treadmill, long walk and motion capture system. The mean of relative root-mean-square errors for gait cycle, stance, stride length, stride velocity and feet ROMs in low and medium velocities, is below 5%. Standard gait parameters for a control group of fourteen healthy young adults are compared with previous studies, showing consistent results. This control group is also compared to a group of eleven unstable adults to determine main discrimination parameters. Significant differences (p < 0.05) are found for cadence, normalized speed, normalized stride length, anterior-posterior feet ROM, medio-lateral hip ROM and medio-lateral hip peak velocity. These results suggest that basic gait parameters, together with feet and hip ROMs and peak velocities, might differentiate unstable subjects from healthy young adults.