The incidence of preeclampsia was reduced among asthmatic patients taking theophylline (one of 85) compared to other asthmatic patients (six of 68, p less than 0.05). Other factors did not appear to account for these results. Theophylline, which is known to inhibit platelet aggregation and alter vascular tone, may reduce the incidence of preeclampsia, at least among asthmatic patients.