Polyvalent immunoglobulin (Ig) therapy has been tested as adjunctive treatment in sepsis and septic shock, but its efficacy is still a matter of debate. This has been explained because clinical trials were mostly performed on small numbers of patients. Moreover, the endogenous level of circulating Ig in patients was never taken into account. In this study, plasmatic Ig classes and protein concentrations were measured at Days (D) 1–2, D3–4 and D5–7 in 62 septic shock patients. At D1–2 as well as at D3–4, patients presented with a significant reduction of plasmatic IgG concentrations. Indeed, at D1–2, 61% of the patients had IgG level below the lowest limit of our age-matched reference values. Plasmatic IgM levels were decreased as well in comparison with reference values from the lab whereas IgA concentrations were not modified. Circulating IgG and IgM concentrations tends to increase overtime. Indeed, at D5–7, most patients (61%) had IgG and IgM levels within the range of normal values. These alterations did not appear to be associated with increased mortality, morbidity or severity after septic shock. However, at D1–2 and D3–4, decreased circulating Ig level was significantly correlated with reduced plasmatic protein concentrations. Overall, our results suggest that an apparent hypogammaglobulinemia is present at D1–2 and D3–4 in septic shock patients, which seems to be related with reduced circulating protein concentration after septic shock. These results need to be confirmed in a larger cohort of patients.