The study concerned 7 subjects (3 men, 4 women) with nonfamilial antithrombin III deficit which, in the absence of known causes of acquired deficiency, was defined as "sporadic". Danazol (an attenuated synthetic androgen) already shown to be capable of compensating for a lack of certain antiproteases, was given in doses of 200 mg 3 times per day for 10 days, resulting in a rapid rise (mean 21.2%) in antithrombin III values. Unlike results reported in the literature in cases of familial antithrombin III deficit, the levels did not drop below their initial values despite discontinuance of the drug but unexpectedly remained high and even shows a slight subsequent rise. Although our study was limited to 11 cases, these data seem worthy of being reported, with the prospect of other confirmations concerning either this sort of "sporadic" antithrombin III deficit or the favorable effect of Danazol, even at long term.