Systemic lupus erythematosus (SLE) is a multi-system chronic inflammatory disease with a broad spectrum of clinical and serological manifestations. Although clinical evidence of proximal skeletal muscle involvement is not rare in patients with SLE, there is no knowledge about the effects of SLE on the macroscopic structural parameters of the muscles. Therefore, in this study, we aimed to explore the muscle strength and structure of SLE patients using isokinetic testing and ultrasonographic imaging. Thirty-one SLE patients (5 men, 26 women) with a mean age of 38.61 +/- 10.68 years and mean disease duration of 3.71 +/- 3.23 years without any previous history of myositis and 31 age- and sex-matched healthy subjects were enrolled. Demographic and clinical characteristics of the patients were recorded. Ultrasonographic evaluations were performed with a 7- to 12-MHz linear probe from vastus lateralis and gastrocnemius muscles of the nondominant extremity. Measurements included muscle thickness, pennation angle, and fascicule length. Isokinetic nondominant knee muscle strength tests were performed at 60 and 180A degrees/s. We found that muscle thickness, pennation angle, and fascicle length of vastus lateralis muscles were increased in patients with SLE in comparison to control subjects (p < 0.001, p = 0.007, and p = 0.217, respectively). On the other hand, the measurements pertaining to the gastrocnemius muscles were found to be similar between the two groups. We observed that all isokinetic knee muscle strength values were decreased in SLE patients, and knee strength values were found to be negatively correlated with age and positively correlated with height (all p < 0.01). We found that the structural and strength changes were seen in the proximal muscles of SLE patients. Pathophysiology and clinical relevance of these changes need to be further investigated.