Pathologies of the blood aggregate state (BAS) as a factor of risk of thromboses and hemorrhages in military pilots constitute the pressing problem of aviation medicine. To study effects of aviation stress on BAS, the following eight models of high-altitude flights were tested: (I) infringed 14-hr flight day (Razinkin model of fatigue), (II) static fight loading, (III) heavy dynamic physical loading, (IV) submaximal dynamic physical loading, (V) complex of submaximal dynamic physical loading, (VI) hyperthermia, (VII) dynamic g-loads during centrifugation, and (VIII) extended altitude hypoxia. The experiments involved 61 essentially healthy males up to 50 years of age who did not take protective pharmaceuticals. Biomedical BAS evaluation included bimodal--coagulolithic (biochemical) and system (mathematical)--monitoring of the test and control samples. The total number of samples was 1432. The investigation resulted in establishment of three forms of BAS-related reaction of organism to the environmental stress factors which are, specifically, hyper-hypocoagulation (thromboses and hemorrhages--II, IV, VI), hypercoagulation (thrombophilia--III) hypo-coagulation (hemorrhages--I, V). On this evidence, it is concluded that stress causes functional strain of the BAS regulation system and consequent thrombohemorrhagic, thrombophilic or hemorrhagic hemoaggregation. Also, the data suggest the fundamental role of BAS in stress pathogenesis and diagnostics of stress-induced diseases in pilots and cosmonauts.