The aims of this study were twofold: (i) to test for possible associations between serological evidence of acute Simkania negevensis (Sn) infection and acute exacerbation of chronic obstructive pulmonary disease and (ii) to examine the prevalence of past infections with Sn in patients with chronic obstructive pulmonary disease. In 120 patients (63%) there was serological evidence of past infection with Sn, which was not significantly different from the rate in a control population. In five hospitalizations serological evidence existed of acute infection with Sn around the time of the exacerbation of chronic obstructive pulmonary disease. In four of these cases, there was serological evidence of acute infection with at least one other respiratory pathogen. It is concluded that Sn can be associated serologically with exacerbation of chronic obstructive pulmonary disease, in most cases together with other respiratory pathogens. The implications of these findings should be investigated further.