Diagnosis of neonatal sepsis may be difficult because clinical presentations are often nonspecific, bacterial cultures are time-consuming and other laboratory tests lack sensitivity and specificity. In this study, we aimed to investigate the role of procalcitonin (PCT), C-reactive protein (CRP), interleukin (IL)-6, IL-8 and tumor necrosis factor-alpha (TNF-alpha) in establishing the diagnosis and evaluating the prognosis of neonatal sepsis. Twenty-six neonates with blood-culture positivity and clinical sepsis, hospitalized for clinical suspicion of neonatal sepsis in neonatal intensive care units of Balcali Hospital, Cukurova University and Adana State Hospital between May 2000 and January 2001 (Group I) and 29 healthy neonates followed at the neonatal units and outpatient clinics of these hospitals (Group II) in the same period were studied. Among the septic neonates, 13 had early-onset (Group Ia) and 13 had late-onset (Group Ib) neonatal sepsis, while 14 of the healthy neonates had perinatal risk factors (Group IIa) and 15 of them had no risk factors (Group IIb). The demographic and clinical characteristics of the septic and healthy neonates were recorded, blood samples for determining serum PCT, CRP, IL-6, IL-8 and TNF-alpha were collected from the healthy and the septic neonates before starting treatment, and these investigations were repeated on the 3(rd) and 7(th) days of treatment. In this study, it was found that: (a) pre-treatment mean serum PCT, CRP, IL-6, IL-8 and TNF-alpha levels were significantly higher in the septic neonates than in the healthy ones, (b) compared with the pre-treatment values, serum PCT, IL-6 and TNF-alpha had progressively decreased on the 3rd and 7th days of the treatment in the 17 recovered patients, though they progressively increased in nine patients who died during treatment, (c) the area under the receiver operating characteristic (ROC) curve (AUC) for PCT, TNF-alpha, IL-6, CRP, and IL-8 were 1.00, 1.00, 0.97, 0.90 and 0.68, respectively. For the cut-off value of PCT >= 0.34 ng/ml, the test was found to have a sensitivity of 100%, specificity of 96.5%, positive predictive value of 96.2%, negative predictive value of 100% and diagnostic efficacy of 98.3% for bacterial sepsis in neonates. For the cut-off value of TNF-alpha >= 7.5 pg/ml, sensitivity, specificity, positive predictive value, negative predictive value and diagnostic efficacy were found to be 100%, 96.6%, 96.2%, 96.5% and 98.3%, respectively. it was detected that sensitivity, specificity and diagnostic efficacy values were lower for IL-6, CRP and IL-8. We conclude that PCT and TNF-alpha are the best markers in the diagnosis of neonatal sepsis, and these markers are also valuable in following the effectiveness of treatment and determining the prognosis of the disease.