The role of bacteria in the pathogenesis of venous ulcers is unclear. It is difficult to be clinically certain of the presence of infection. Routine bacteriology is often unhelpful, and any simple investigation that improves diagnosis in this situation would be of value. C-reactive protein (CRP) levels are useful in detecting infection in other situations, and they may be of value in this context as well. C-reactive protein levels were measured in 50 patients with venous leg ulcers and in 20 patients with active venous eczema. There was no elevation of CRP levels in patients with eczema alone, nor in the majority of patients with ulcers. Sixteen patients had raised CRP levels: 7 had clinically obvious infection, and 9 had erythematous skin of uncertain cause surrounding their ulcers. All had positive bacterial cultures from the ulcer base, with β-hemolytic streptococci the main contaminant. Treatment with an appropriate antibiotic reduced CRP levels to normal, cleared the bacteria from the ulcers, and was associated with resolution of erythema. CRP levels appear to distinguish between infectious and inflammatory causes of erythema in patients with gravitational disease.