The aim of this study was to investigate NUT (nuclear protein in the testis) expression in ovarian germ cell tumours (GCTs). Immunostaining for NUT protein was performed in 10 mature cystic teratomas and in 49 malignant ovarian GCTs including 15 pure dysgerminomas, six dysgerminomas associated with gonadoblastoma, nine yolk sac tumours, 12 immature teratomas, and seven mixed malignant tumours. Only nuclear staining was considered a positive finding although cytoplasmic staining was noted when present. Thirty-seven (76%) malignant GCTs were NUT positive but staining was usually of weak to moderate intensity and observed in a relatively small proportion of neoplastic cells. Staining in immature teratomas and yolk sac tumours was restricted to foci of hepatoid and intestinal/glandular differentiation, where both nuclear and cytoplasmic reactivity were observed. In dysgerminoma associated with gonadoblastoma only the in situ and invasive germ cell elements were NUT positive. Nuclear staining was not seen in benign teratomas. Most malignant ovarian GCTs express NUT protein, albeit focally, and this should be considered when evaluating immunostaining in the differential diagnosis of poorly differentiated malignancies, particularly NUT midline carcinoma. Since NUT protein appears to play a role in normal germ cell maturation it may influence intestinal or hepatoid differentiation within malignant GCTs.