From among a population of same-sexed Danish twins born 1870-1920 a total of seventy-eight probands with process-schizophrenia were selected. Among the co-twins of twenty-five MZ probands nine were found to have schizophrenia, and a further five co-twins had presented schizophrenic traits; the corresponding figures for the co-twins of forty-five DZ probands were eight and five respectively. In eight probands zygosity could not be determined with sufficient certainty. If partners with schizophreniform, paranoid or atypical psychoses are accepted as concordant, the probandwise MZ and DZ concordance rates will be 0.56 and 0.26. This difference is significant, which in agreement with earlier twin studies indicates that genetic factors influence population variability with regard to schizophrenia. The actual value of the MZ concordance rate in the present study is, however, in the lower range of all previously obtained figures, in this respect corresponding with two recent Scandinavian studies. It is characteristic of these three studies as opposed to all others that the probands have been selected from a total population.