The aim of our study was to prospectively examine adrenal function, including cosyntropin stimulation, and its prognostic value in patients with moderate community-acquired pneumonia (CAP). 59 consecutive adult patients hospitalised on normal wards because of CAP were enrolled. A cosyntropin stimulation test was performed and serum concentrations of C-reactive protein, procalcitonin, interleukin-6, tumour necrosis factor-α, ACTH, cortisol, dehydroepiandrosterone (DHEA) and dehydroepiandrosterone-sulfate (DHEAS) were measured. Predefined outcome parameters were clinical instability after 72 h, mortality and combined intensive care unit (ICU) admission or mortality. Critical illness-related corticosteroid insufficiency (CIRCI) occurred in six patients (10.3%). Cortisol, age-corrected DHEA, ACTH and the DHEA/DHEAS ratio were elevated in patients remaining unstable after 72 h. In multivariate analysis, cortisol (p = 0.03), ACTH (p = 0.04) and the pneumonia severity index (PSI) score (p = 0.005) independently predicted clinical instability after 72 h, and only cortisol predicted mortality (p = 0.04) and combined ICU-admission or mortality (p = 0.006). The predictive value of serum cortisol after receiver operating characteristic curve analysis equalled that of the PSI score. Patients with serum cortisol >734 nmol·L(-1) had a high probability for mortality (OR 38.3; p = 0.002). CIRCI is rare in patients with moderate CAP. Adrenal function is related to the prognosis of CAP. The diagnostic accuracy of serum cortisol equals that of the PSI score. Serum cortisol should be evaluated within clinical prediction scores on larger studies.