Fifteen families with clustering of infective dermatitis associated with HTLV-1 (IDH) and/or HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP) were observed among 28 families of IDH index cases, 93% of them occurring in two generations. With the exception of two mothers of children with IDH, all the mothers with HAM/TSP had at least one child with HAM/TSP. This is the first report of such clustering involving many families.