The Beginning Breastfeeding Survey-Cumulative (BBSC) is an instrument that assesses the overall maternal perception of breastfeeding effectiveness during the early postpartum period. The aims of this study were to adapt and validate a Spanish version of the BBSC and provide new evidence regarding its validity. A standard forward and back-translation process was used to obtain the Spanish version of the questionnaire (BBSC-E). A sample of 793 breastfeeding mothers was used. Data were obtained from clinical records and questionnaires self-administered at discharge and at 1-4 months postpartum. The factorial structure of the BBSC-E was examined with both an exploratory and confirmatory approach. Internal consistency, comparisons between known groups, and correlations with other variables were also assessed, and receiver operating characteristics (ROC) curves were used to determine its predictive validity. Analysis revealed a two-dimensional structure, with infant and maternal subscales, and adequate fit statistics. The Root Mean Squared Error of Approximation (RMSEA) was .067. The Cronbach's α of the overall scale was .91. There were statistically significant associations between the BBSC-E and (1) previous breastfeeding experience, early skin-to-skin contact, exclusive breastfeeding, breastfeeding difficulty and self-efficacy, and infant weight loss during hospitalization and (2) at 1-4 months postpartum, with the breastfeeding status, the intensity of breastfeeding problems, and satisfaction with breastfeeding. The total BBSC-E area under the ROC curve for any breastfeeding at 60 days postpartum was .73, and a score of 88 was the most discriminative cut-off point for maximum performance. The BBSC-E is a reliable and valid instrument for measuring breastfeeding effectiveness during early postpartum.