Of 21 dogs from Sapporo, Hokkaido that had been recognised as having been bitten by ticks, 16 were seropositive to Borrelia burgdorferi by ELISA. Thirteen of the seropositive dogs showed signs such as fever, astasia, convulsions, anorexia, fatigue, abnormal gait, nervous signs, diarrhoea, corneal opacity and conjunctivitis. These signs subsided as a result of antibiotic treatment within five days. The plasma concentrations of creatinine in the 21 dogs were higher than in control dogs. Seven ticks that were removed from seven of the dogs were Ixodes persulcatus, and B burgdorferi was isolated from the midgut of two of the ticks.