We studied 132 children admitted consecutively with their first febrile convulsion to assess whether the degree of fever at the onset of the convulsion can predict the risk of subsequent convulsions. The children studied were reviewed at least 2 years after the initial febrile convulsion to determine the number of children who had recurrences of febrile convulsions and/or afebrile convulsions. Children with body temperatures below 39 degrees C at the onset of their initial febrile convulsion (Group 1) were two and half times more likely to experience multiple convulsions within the same illness than those with body temperatures above 39 degrees C (Group 2). This occurred when the body temperature rose above that which had triggered the initial febrile convulsion. Children in Group 1 were also over three times more likely to experience recurrent febrile convulsion in subsequent illnesses than those in Group 2. As for subsequent development of afebrile convulsion or epilepsy, although the risk was low, it only occurred in Group 1. It is suggested that the known association between multiple convulsions, recurrent febrile convulsions and epilepsy may be due to the single predisposing factor of a low degree of fever at the onset of febrile convulsion. Each child with febrile convulsion may have his own threshold for eliciting a convulsion with fever; the lower this threshold is, the more likely are subsequent convulsions.