Among 145 patients with the complex of myxomas, spotty pigmentation, endocrine overactivity, and psammomatous melanotic schwannomas, 31 (21%) had mammary lesions. The ages of these 26 females and five males ranged from 6 to 64 years (mean, 30 years). Five patients had breast symptoms. In 21 (81%) of the females, benign mesenchymal lesion(s) were detected pathologically. These were characterized by accumulations of large amounts of ground substance in the lobules that alterated the stroma to a very loose and myxoid tissue. The change involved single lobules (lobular myxoid change), small groups of lobules (nodular myxoid change), and large aggregates of lobules (myxoid fibroadenoma); the interlobular stroma was affected to a lesser degree. The lesions were multicentric and bilateral in eight patients (38%). Because the myxoid breast lesions were familial, were frequent findings in the complex, and were similar histologically to the cardiac and cutaneous myxomas in the complex, they undoubtedly are a component and a pathologic marker of the complex. They were the presenting feature of the complex in six patients (19%). Therefore, discovery of the myxoid breast lesions on pathologic examination should raise suspicion of the complex, and affected patients (and their primary relatives) should be evaluated accordingly.