Aplastic crisis in sickle cell anaemia has been associated with infection by a serum parvovirus-like virus (SPLV). Reports of outbreaks of these crises in hereditary spherocytosis (HS) were investigated to see if the data were consistent with a single agent such as SPLV being their cause. The age distribution of the cases and the pattern of spread were compatible with an infectious aetiology. The symptoms and duration of the cases, and the intervals between them, were sufficiently uniform to indicate that they were caused wholly or predominantly by a single agent with an incubation period of about nine days. There were close similarities between these illnesses and recognised cases of SPLV infection. It is concluded that SPLV is probably the main cause of aplastic crisis in HS.