Beta-endorphin is released during stress and activates receptors in ncl. tractus solitarius and induces bradycardia and respiratory depression.We measured BEI in CSF. in caudal area of ncl. tractus solitarius, brainstem gliosis by immunohistochemistry and HX in corpus vitreum liquor in SIDS victims (n=27). The levels were compared with control infants dying from other causes than SIDS (n=12).BEI measurements in CSF revealed two distinct subpopulations of SIDS, P<0.01, (G1 and G2). In approximately half of the SIDS victims (G1) there was a high level of BEI in CSF (160-400 fmol/ml CSF) and a non detectable level of BEI in ncl. tractus solitarius area (<4.3 fmol/mg tissue), p<0.05, G1 compared to G2. G1 cases had a higher level of astrogliosis in ncl. Oliva inf., p<0.01, than G2 patients. The BEI level in CSF of G1 correlated with HX concentrations in corpus vitreum liquor, r=0.92, p<0.001. The other half of SIDS cases (G2) had non delectable level of BEI in CSF (<4.3 fmol/ml) and high level of BEI in ncl. tractus solitarius area, (<4.3-17.1 fmol/mg tissue). The controls with high beta-endorphin level in CSF died under stressfull circumstances as heart operation and the controls dying from infection had low BEI in CSF.Conclusion; SIDS is characterized by 2 groups. G1: High BEI level in CSF, low level BEI in ncl. tractus solitarius area, increased level of brainstem gliosis and a correlation between BEI level in CSF and HX level in corpus vitreum liquor. Group 2: Low BEI in CSF, high level BEI in ncl tractus solitarius area and low level of brainstem gliosis.