A retrospective study was performed in 32 children with hemophagocytic lymphohistiocytosis, 16 of whom had affected siblings. Altogether 22 of these children, of whom the majority (13/22) were familial cases, had clinical or laboratory signs of infection. Laboratory analysis demonstrated Epstein-Barr virus in five children, cytomegalovirus in three and human parvovirus in two. Two siblings with onset of familial hemophagocytic lymphohistiocytosis within one month of each other, both of whom demonstrated serological indications of a recent human parvoviral infection at onset, are described. It is concluded that a viral infection cannot serve as the sole criterion for distinguishing a virus-associated hemophagocytic syndrome as an entity separate from familial hemophagocytic lymphohistiocytosis. Instead, it is suggested that viral infections may elicit a bout of the familial hemophagocytic lymphohistiocytosis disorder in genetically predisposed individuals.