For diagnosis of equine granulocytic ehrlichiosis (EGE) serological testing of antibodies to Ehrlichia equi is frequently used. An elevated antibody level is often misinterpreted as confirmative of active infection and results in treatment with antibiotics. If only seropositivity is considered as the diagnostic criterium, many horses showing convalescence titres will be treated. This study was undertaken to obtain information about the kinetics of antibodies during the course of infection and, for this purpose, 45 horses with clinical signs of EGE and confirmed ehrlichiaemia were monitored serologically and clinically over time. For a correct handling of cases with suspected EGE, the following results should be helpful: (i) 44% of the horses in the acute ehrlichiaemic stage were found to be serologically positive to E. equi; (ii) all horses showed a rapid increase in antibody titre, reaching maximum value within a month after the ehrlichiaemic stage; (iii) when 8 months had passed, titres had decreased, but 18 of 24 examined horses were still serologically positive; (iv) after 12-15 months most of the horses (n = 10) were serologically negative; and (v) the period required for complete clinical recovery varied from one day up to 6 months after antibiotic treatment.