Cortical thickening and lucency in the proximal humerus are uncommon radiological findings. Pseudotumor deltoideus was reported for the first time in the literature in 2001 in a study of five cases of asymptomatic, subacute or chronic, and painful pseudotumor deltoideus in the deltoid insertion; it was described as benign in character and possessing various anatomic variations not previously defined. In this paper, by presenting the clinical and radiological findings of a female patient with acute pain in the left shoulder with radiological images indicative of the condition, to the best of our knowledge, this study constitutes the second report in literature and the first case from Turkey of pseudotumor deltoideus. A 30year-old female presented with an acute-onset of severe pain in the left shoulder. Cortical thickening, irregularity and intracortical lucency were observed at the deltoid insertion. In case of cortical thickening in the proximal humerus deltoid insertion accompanied by irregularity and intracortical lucency associated with acute onset of severe lateral shoulder pain, pseudotumor deltoideus should be considered as a diagnosis. MRI is helpful in the differential diagnosis