Aim To determine the diagnostic value of serological infection markers and body temperature in discriminating complicated from uncomplicated diverticulitis. Methods Patients in whom diverticulitis was pathologically or radiologically proven at presentation were included. Patients were classified as either complicated (Hinchey Ib, II, III and IV) or uncomplicated (Hinchey Ia) diverticulitis. The discriminative value of C‐reactive protein (CRP), white blood cell (WBC) count and body temperature at presentation was tested. Results A total of 426 patients were included in this study of which 364 (85%) presented with uncomplicated and 62 (15%) with complicated diverticulitis. Only CRP was of sufficient diagnostic value (area under the curve 0.715). The median CRP in patients with complicated diverticulitis was significantly higher than in patients with uncomplicated disease (224 mg/l, range 99–284 vs 87 mg/l, range 48–151). Patients with a CRP of 25 mg/l had a 15% chance of having complicated diverticulitis. This increased from 23% at a CRP value of 100 mg/l to 47% for 250 mg/l or higher. The optimal threshold was reached at 175 mg/l with a positive predictive value of 36%, negative predictive value of 92%, sensitivity of 61% and a specificity of 82%. Conclusion WBC count and body temperature are of no value in discriminating complicated from uncomplicated diverticulitis. Only CRP can be used as an indicator for the presence of complications, but a low CRP does not mean that complicated disease can safely be excluded. Therefore, radiological examination remains central in the diagnostic work‐up of patients presenting with diverticulitis.