©2016 AACR. Purpose: Severe skeletal muscle (SM) loss (sarcopenia) is associated with poor cancer outcomes, including reduced survival and increased toxicity. This study investigates SM measures in metastatic breast cancer (MBC) patients receiving first-line taxane-based chemotherapy and evaluates associations with treatment toxicity and other outcomes. Experimental Design: Using computerized tomography (CT) images taken for the evaluation of disease burden, skeletal muscle area (SMA), and density (SMD) were measured at the third lumbar vertebrae. Sarcopenia was defined as skeletal muscle index (SMI = SMA/height2) ≤ 41. Skeletal muscle gauge (SMG) was created by multiplying SMI x SMD. Fisher exact tests, t tests, the Kaplan-Meier method, and Cox regression modeling were used. Results: MBC patients (N = 40), median age 55 (range, 34-80), 58% sarcopenic, median SMG 1296 AU (SD, 522). Grade 3-4 toxicity was found in 57% of sarcopenic versus 18% of nonsarcopenic patients (P = 0.02). Toxicity-related hospitalizations were also higher in sarcopenic patients (39% vs. 0%, P = 0.005) as were any adverse events-defined as any grade 3-4 toxicities, hospitalizations, dose reductions, or dose delay-(74% vs. 35%, P = 0.02). Low SMG was associated with grade 3-4 toxicity (P = 0.04), hospitalization (P = 0.01), and time to treatment failure (for progression or toxicity; P = 0.03). Low SMG had a borderline significant association with any adverse event (P = 0.06) and overall survival (P = 0.07). Conclusions: SM measures are associated with toxicity outcomes and survival in MBC patients receiving first-line taxane-based chemotherapy. Further studies are needed to explore how routinely obtained CT scans can be used to individualize dosing and improve treatment planning.