Objective: This study examined test-retest reliability, internal consistency, and construct and predictive validity of the Physical Activity and Nutrition Self-Efficacy (PANSE) scale, an 11-item instrument to assess weight-loss self-efficacy among postpartum women of lower income. Methods: Seventy-one women completed the PANSE scale and psychosocial measures at baseline and at 7 and 13 weeks of a pilot study of weight loss. Results: Test-retest reliability of PANSE scores was r = 0.55 (P less than 0.01). Cronbach alpha was r = 0.89. Construct validity was supported by correlations with less-healthful behaviors (r = -0.33, P = 0.005), perceived stress (r = -0.24, P = 0.04), and decisional balance (r = 0.25, P = 0.03). PANSE change scores correlated with intervention group weight changes (r = -0.54, P less than 0.01), although baseline scores did not. One factor emerged (loadings 0.80 to 0.57). Conclusions and Implications: The PANSE scale demonstrated adequate reliability, internal consistency, construct validity, and moderate predictive validity. Replication is needed to confirm study findings. (Contains 1 table.)