Three-dimensional (3D) US is a new sonographic modality which represents a promising tool in the assessment of joint and periarticular tissues abnormalities in rheumatic diseases. The available literature has recently underlined its advantages mainly related to the virtual operator independence due to image acquisition of infinite 3D data sets obtained by transducer automated sweeping. Shortening of the US examination time represents another notable advantage over conventional two-dimensional (2D) US. The aim of the present study was to investigate the validity of 3D US in assessing Achilles tendon enthesitis by comparing it with 2D US. US examinations were performed by using a Logiq 9 (General Electrics Medical Systems, Milwaukee, WI) equipment with a high-frequency (8-15 MHz) volumetric probe. One hundred and eighty-six Achilles tendon enthesis of 93 SpA patients were examined. The analysis of each basic US finding demonstrated from good to excellent agreement rates between 3D and 2D US, both in dichotomous assessment of sonographic lesions and in the use of semi-quantitative grading. Excellent agreement between the two modalities was demonstrated in the assessment of both inflammatory changes and structural lesions. Our study for the first time demonstrated that 3D US is a valid imaging modality for the assessment of Achilles tendon enthesitis.