This paper describes a novel method and system to monitor and compute swimmer performance. We placed a 3-axes accelerometer onto the athlete's back. This accelerometer allows for detecting and interpret single gestures during the training session. The acquired data can be processed to compute synthetic parameters for a qualitative assessment of the performance. Symmetry and no. of foot-/arm-stroke integrate the measurement. 3 healthy volunteers (1 male, 2 females) participated to the system validation. The system proved a fine detection of each phase and movement in the analyzed sport. Also comfort and wearability was positively assessed.