Yolk sac tumors may exhibit numerous patterns. One that has received little attention overall, yet is not uncommon, is a solid pattern, which is especially prone to misinterpretation, usually as seminoma, in biopsy specimens from metastatic or mediastinal sites. This distinction is of critical importance as the 2 tumors are treated differently. To determine features useful in the diagnosis of solid yolk sac tumor we reviewed 52 germ cell tumors (28 testicular primaries, 21 metastases from the testis, and 3 mediastinal primaries) that had a yolk sac tumor component with foci of solid growth, defined as a sheet-like arrangement of tumor cells occupying >2 mm and with no or only rare microcysts. Solid yolk sac tumor was almost always associated with other patterns, most commonly microcystic/reticular (75%), glandular (35%), and myxoid (25%). The solid foci consisted of sheets of cells with usually abundant cytoplasm that was mostly (85%) pale to clear and frequently had intercellular basement membrane deposits (75%), rare microcysts (67%), significant nuclear pleomorphism (65%), and hyaline globules (65%). In 2 cases (4%), the cells were small with scant cytoplasm (blastema-like variant). A myxoid background (39%), lymphocytic infiltrate (17%), and an appliqué pattern (8%) were sometimes observed. On immunostaining, AE1/AE3 cytokeratin and glypican 3 provided the most intense and diffuse reactivity for solid yolk sac tumor, whereas