The incidence of Lyme borreliosis was studied prospectively in 165 patients with acute idiopathic sensorineural hearing loss. The prevalence of positive levels of antibodies against Borrelia burgdorferi was sixfold higher in patients with sensorineural hearing loss than in the general population in Finland. Four patients fulfilled the criteria for Lyme borreliosis. No specific risk factors were found with which to predict the occurrence of Lyme borreliosis among patients with hearing loss. In logistic regression modeling the poor outcome of hearing loss was best explained by advanced age, high-frequency or flat-type hearing loss, and absence of positive levels of antibodies against B. burgdorferi. Although the causal relationship between Lyme borreliosis and sensorineural hearing loss is difficult to verify, we suggest that Lyme borreliosis is a rare but potentially treatable cause of sudden deafness. We propose that in endemic areas antibodies against Lyme borreliosis should be determined in patients with idiopathic sensorineural hearing loss.