Caterpillar bodies are eosinophilic, periodic acid-Schiff (PAS)-positive globules arranged in a linear fashion in the epidermis overlying subepidermal blisters of porphyria cutanea tarda (Am J Dermatopathol 1993;15:199-202). We retrospectively studied by transmission electron microscopy nine cases of porphyria cutanea tarda (PCT) that demonstrated caterpillar bodies. We identified three components of the eosinophilic bodies: degenerating keratinocytes, colloid bodies, and basement membrane bodies. The colloid bodies consisted of whorled masses of filaments containing degenerating melanosomes, vacuoles, mitochondria, and desmosomes. Basement membrane bodies were composed of convoluted basement membrane material and associated collagen. Both colloid and basement membrane bodies were often associated with degenerating keratinocytes, were located both intra-and extracellularly, and were occasionally fused to one another. We believe that caterpillar bodies are a combination of degenerating keratinocytes, colloid bodies, and basement membrane bodies formed by repeated blistering and reepithelialization with transepidermal migration. Furthermore, we believe that caterpillar bodies are a diagnostic clue for the diagnosis of PCT.