In this study, most individuals presented with a slowly growing, symptomless, subcutaneous mass although rarely these masses were rapidly growing, and/or were tender, painful, warm, and/or were accompanied by skin changes, pigmentation, sweat gland enlargement, and/or increased hair overlaying the tumor. Sixty-eight percent of these cases were diagnosed in the first year of life, 23% were diagnoses at birth, and 9% were diagnosed in children 2–5 years old. The male-to female ratio was 2.4. The tumors were most common in the axilla (23% of cases), upper arm (12.5%), lower arm (10.5%), external genitalia area (9.5%),  and inguinal region (7.5%); less common or rare sites for these tumors were the buttocks, back of the head, back of the neck, back of the torso, lower arm, leg, foot, chest, and anal area. Most cases presented as solitary masses but 3 cases presenting with 2 tumors and 1 case presenting with 5 tumors. Overall, the size of these tumors varied between 0.5 and 4.0 cm but in rare cases were as large as 20 cm. Two subsequent studies of 145 and 60 individuals agreed with all of these results but also did diagnose  FHI in individuals as old as 8 and 14 years, respectively. Pathology
On gross pathological examination, FHI tumors are soft, poorly demarcated, fibro-fatty masses located in subcutaneous tissues. These tumor masses tend to blend with the surrounding fat tissues, may be lobulated, and in rare cases, especially when large, extend into nearby muscles, nerves, and/or fibrous/connective tissue structures.
Microscopic histopathological analyses of hematoxylin and eosin stained FHI tumor tissues consistently reveal three very different component zones in virtually all cases: 1) haphazardly arranged, intersecting bundles of bland fibroblast-like/myofibroblast-like cells; 2) mature adipose (i.e. fat) tissue; and 3) highly vascular, myxoid (i.e. connective tissue appearing more blue or purple than normal connective) nodules of primitive-appearing spindled-shaped to star-shaped cells. Chronic inflammatory cell (e.g. lymphocytes, macrophages, and plasma cells) aggregates are commonly present in these lesion. The relative proportion of fibroblast/myofibroblast, adipose tissue, and myxoid zones commonly vary from case to case.Earlier Immunohistochemical analyses of FHI tumor tissues in a small number of cases have given varying results. A more recent and larger study reported that 75% of tested cases showed variable expression of smooth muscle actin proteins by the cells located primarily in fibroblast/myofibroblast zones and occasionally by the cells located in myxoid zones; the S100 protein was expressed by the fat cells of the adipose tissue zone in all cases; CD34 protein was expressed by the cells in myxoid and fibroblast/myofibroblast zones in all cases; and cells expressing the desmin protein were not detected in any case. A smaller, more recent study also found that 8 of 13 FHI cases had tumor cells in the fibroblast/myofibrobalast zone that expressed smooth-muscle actin protein, 6 of 8 cases had tumor cells in the myxoid zone that expressed the CD34 protein, and 7 of 7 cases had tumor cells in the adipose tissue zone express the S100 protein.