We previously showed 1-month of Transcutaneous Electrical Nerve Stimulation (TENS) reduces movement-evoked pain and fatigue in women with fibromyalgia (FM). Using data from this study (Fibromyalgia Activity Study with TENS, FAST), we performed a responder analysis to identify predictors of clinical improvement in pain and fatigue with TENS, validated these models using Receiver-Operator-Curves (ROC), and determined number-needed-to-treat (NNT) and number-needed-to-harm (NNH). Participants were randomly assigned to active-TENS (2-125Hz; highest-tolerable intensity), placebo-TENS, or no-TENS for 1-month. At the end of the randomized phase, placebo-TENS and no-TENS groups received active-TENS for 1-month. The predictor model was developed using data from the randomized phase for the active-TENS group (n=103) and validated using data from placebo-TENS and no-TENS groups after active-TENS for 1-month (n=155). Participant characteristics, initial response to TENS for pain and fatigue, sleep, psychological factors, and function were screened for association with changes in pain or fatigue using a logistic regression model. Predictors of clinical improvement in pain were initial response to pain and widespread pain index (AUC was 0.80; 95%CI: 0.73,0.87). Predictors of clinical improvement in fatigue were marital status, sleep impairment and initial response to TENS (AUC was 0.67; 95%CI: 0.58,0.75). NNT for pain and fatigue ranged between 3.3-5.3. NNH ranged from 20-100 for minor TENS-related adverse events. The response to an initial 30-minute TENS treatment predicts who responds to longer-term TENS use in women with FM, making this a clinically useful procedure. NNT and NNH suggests TENS is effective and safe for managing pain and fatigue in FM.