Patients with active sarcoidosis show minor or no elevation of serum C-reactive protein. During the research of the significance of measurement of this acute reactant in patients with active sarcoidosis, we noticed that during the intercurrent infection the levels of CRP rise significantly. This report shows that this observation is not an accidental event, but a rule. With this purpose in mind, we compared the values of serum CRP in 9 patients with active sarcoidosis during intercurrent infection and 17 with active sarcoidosis only. The difference was statistically significant; high levels were found in the sera of patients with intercurrent infection. Therefore, we consider this simple test a helpful means in the diagnosis of intercurrent infection in patients with active sarcoidosis.