When dermatofibromas are composed predominantly of fibroblasts and extend into the subcutaneous tissue, it may be difficult to distinguish them from dermatofibrosarcoma protuberans. Because the patterns of extension of dermatofibroma have not been well characterized, we studied 185 cases of the fibrous variant of dermatofibroma with extension into the subcutaneous tissue and 40 cases of dermatofibrosarcoma protuberans. Dermatofibromas had two main patterns of extension into subcutaneous tissue. One pattern, seen in 133 of 185 cases (72%), consisted of irregular extension into the subcutaneous tissue in a vertical or radial fashion, predominantly along the septa, which appeared wedge-shaped. The other pattern, seen in 52 of 185 cases (28%), showed a smooth and well-demarcated deep margin that bulged into the subcutaneous tissue. Dermatofibrosarcoma protuberans also had two main patterns of extension into the subcutaneous tissue. In one pattern, seen in 12 of 40 cases (30%), slender spindle-shaped cells extended along septa and between fat cells in a classic honeycomb or lacelike pattern. The other pattern observed in 24 of 40 cases (60%) exhibited a distinct multilayered pattern in which the bundles of slender spindle-shaped cells showed a predominantly parallel orientation to the skin surface. In four cases (10%), a mixture of both patterns was present. We conclude that the patterns of extension of dermatofibroma into the subcutaneous tissue are different from the patterns of dermatofibrosarcoma protuberans.