We reviewed serological and epidemiological data relating to 1000 consecutive patients from whom specimens were submitted for estimation of bartonella antibodies, using MRL Diagnostics Bartonella IFA IgM and IgG kits. Using 289 control sera, we estimated the specificity of the kits as [ges ]99·0%. Evidence of bartonella infection was found in 16·3% of patients examined. Rates varied by patient group: 20% of patients for whom a diagnosis of cat scratch disease (CSD) was considered probable had evidence of infection, as did 10·4% of patients with ‘possible CSD’, 8·1% of patients with possible bacillary angiomatosis, 18·2% of patients with ‘culture negative’ endocarditis and 17·6% of patients with possible bartonellosis with ophthalmic involvement. An IgM response was seen in 6·6% of patients and IgG in 15·1%. Cases were more frequent among males than females (18·5% vs. 13·9%). Analysis by age showed that although rates of infection were highest in the decades 0–9 years (19·4%) and 10–19 years (20·7%), they fell only slightly in the next three decades. MRL bartonella kits appears to provide a useful and specific approach to the diagnosis of these infections.