Numerous fibrous structures stainable with resorcin--fuchsin were found by light microscopy in five pulmonary hamartomas. Their electronoptic correlates were granular dense trabecular formations reminiscent of oxytalan fibres whereas trabeculae typical of elastica were rare. The trabecular material orientation according to collagenous fibrils as well as its occasional transverse striation suggest a special form of collagen to be involved.