A recent unexpected finding of inhibited platelet aggregation in response to a single (1.64 mmol/l) dose of arachidonic acid (AA), during the relapse of childhood nephrosis, prompted us to assess aggregation in response to multiple doses of AA: 1.64,0.82,0.41,0.20 mmol/l, in two groups of children, in the relapse (n = 34) or remission (n=41) phase of nephrotic syndrome. During relapse: the highest dose of AA (1.64 mmol/l) evoked reversible and inhibited aggregation in 91% of patients. However, at the lower doses there were enhanced responses as measured by both maximum aggregation (%) and slopes of the aggregation curves. In contrast, during remission, irreversible aggregation was obtained at the highest AA dose, while at the lowest two doses (0.41 and 0.20 mmol/l), no aggregation responses were obtained in 4 (9%) and 7 (17%) patients respectively; in those who responded there was a long lag phase. Healthy controls (n = 21) exhibited their highest responses to 1.64 and 0.82 mmol/l AA and at the lowest AA doses (0.41 and 0.20 mmol/l), a total absence of responses was noted in 40% and 71% of samples respectively. We conclude that during relapse platelet sensitivity, as shown by irreversible aggregation in response to multiple AA doses, shifts towards the lower doses, when compared with healthy controls; while during remission responses fall in-between the relapse and control groups, indicating the maintenance of platelet sensitivity during this phase of nephrosis.