However, we also observed that between 1955 and 1959 there was a pronounced seasonal variation in births of schizophrenia patients but not their normal siblings. On the basis of these findings, we suggested that the seasonal variation in schizophrenia births may consist of two components: a constant component caused by parental procreational habits and an irregular component caused by environmental risk factors that, when they occur, considerably increase the magnitude of the seasonal variation in births among patients but not their siblings and are genuine risk factors for schizophrenia. Infections caused by B. burgdorferi are one possible explanation for this irregular component. However, in Finland, Lyme borreliosis is endemic in southwestern coastal areas, in which the incidence of schizophrenia is lowest. In many northeastern areas, in which the incidence of schizophrenia is exceptionally high, ticks no longer survive (1–3).