Objective: This study aimed to analyze serving sizes declared on food labels with respect to their conformity to Brazilian law, variability among similar foods, and relationship to energy values. Methods: Food label information from 24 groups of processed foods was collected in one supermarket in southern Brazil. Declared serving sizes were classified into five groups according to their level of compliance with those recommended by law: <70% and >130% inadequate and 70-99%, 100% and 101-130% adequate. Descriptive analyses, Spearman correlations, and Kruskal-Wallis tests were applied. Results: A total of 1,953 food products were analyzed, of which 72% reported serving sizes exactly equal to those recommended, and 14% fell within the acceptable 60% range established by law. Even though most foods complied with the law, there was a variation in declared serving sizes among similar foods, with the largest ranges observed for hamburger and meatballs (364 g). In addition, declared serving sizes were significantly and positively correlated to energy value in most food groups, revealing that smaller serving sizes may be used to report lower energy value. Conclusion: The 60% acceptable serving size range established by law may lead to a lack of serving size standardization and significant variability in declared energy values on labels. This can potentially make labels confusing when comparing similar products and thus affect food choices. It is necessary to revise Brazilian law so that serving sizes are standardized among similar foods in order to facilitate healthy food choices.