The first major family study of schizophrenia, reported by Ernst Rüdin in 1916, examined 2,732 siblings of 755 probands, diagnosed according to the teachings of Kraepelin. This study, the goal of which was to see whether the segregation pattern of schizophrenia in siblings conformed to simple mendelian expectations, was the first in psychiatry to use systematic ascertainment, proband correction and calculation of an age corrected risk of illness--the morbid risk (MR). The MR for narrowly and broadly defined schizophrenia in this sample can be calculated to equal 5.4 and 7.7%. "Other psychoses"--a heterogeneous category--were also common in these siblings (a MR of 5.1%). In a small sample of half-siblings, the MR for narrowly defined schizophrenia was quite low (0.6%). The risk for schizophrenia in siblings was significantly increased by a parental diagnosis of alcoholism, a history of schizophrenia in second or third degree relatives, and, particularly, by a parental diagnosis of "other psychoses." No evidence was found for sex-specific transmission of schizophrenia in these sibships. The MR for narrowly and broadly definite schizophrenia in parents of these probands can be estimated to be 2.3% and 3.9%, respectively. In accord with more recent studies, Rüdin found i) a familial relationship between schizophrenia and other psychoses ii) a substantially lower risk for schizophrenia in parents vs. siblings and iii) a segregation pattern of schizophrenia in siblings that did not conform to that expected for a simple mendelian disorder.