Background: In August 2010, the U.S. Food and Drug Administration issued draft guidelines for developing antibiotics for treating acute bacterial skin and skin structure infections (ABSSSI), with the outcome measure of such treatment relying primarily on the cessation of spread or on the decrease in size of skin lesions at 48-72 h after the initiation of such treatment. In 2012, the Foundation for the National Institutes of Health proposed an interim outcome measure defined as a reduction in lesion size by ‡ 20% at a 48-72 h examination. These recent changes make it necessary to identify reliable methods for measuring the lesions in acute infections of the skin. Methods: In the first study of the Efficacy and Safety of 6-day Oral Tedizolid in Acute Bacterial Skin and Skin Structure Infections vs. 10-day Oral Linezolid Therapy (ESTABLISH-1), the sizes of ABSSSI lesions were evaluated with the two methodologies of: (1) Digital planimetry (DP) of photographed lesions, and (2) a ruler technique (RT) with measurement of the longest head-to-toe length and greatest perpendicular width of lesions, to compare the respective response rates of lesions to the two antimicrobial regimens in the study. Results: The RT method and DP showed similar percentages of subjects in which treatment stopped the spread of ABSSSI lesions (93.2% vs. 94.2%, respectively) but showed less agreement for a reduction in lesion size, of ‡ 20% (87.7% vs. 62.0%, respectively) across all categories of lesions (cellulitis/erysipelas, major cutaneous abscess, and acute wound infection) at 48-72 h after the initiation of treatment. Conclusion:The results of the ESTABLISH-1 study show that both the RT method and DP are consistent and reliable techniques for measuring the sizes of ABSSSI lesions. Ultimately, changes in lesion size, rather than the absolute value of lesion size, will be used to assess the outcomes of treatment for ABSSSI in clinical research.