Pseudoangiomatous stromal hyperplasia (PASH) is a benign proliferation of mammary stroma mostly described as an incidental microscopic finding. Clinically, it can manifest as a palpable, well-circumscribed breast mass or in rare cases as a diffuse bilateral process causing massive breast enlargement. The most postulated theory for aetiology of this condition is hormonal stimulation of mammary myofibroblasts, particularly by progesterone. A definite diagnosis of PASH is based on typical pathological findings like stromal hyperplasia and empty slit-like channels positive for myofibroblastic and negative for endothelial markers. The main clinical differential diagnosis is a fibroadedoma or phylloid tumour, and histologically a low-grade angiosarcoma. There are less than 200 cases of tumorous PASH and less than 20 of diffuse PASH reported so far. Here we present a case report of huge diffuse PASH, that is, to our knowledge, the first in a pregnant woman.