It has been proposed that the nailfold subpapillary plexus visibility score (PVS) may be a marker of susceptibility to schizophrenia. To further investigate this hypothesis, we evaluated plexus visibility in a sample of 61 chronic schizophrenics, a large sample of first-episode psychotic patients and their first-degree relatives (50 with schizophrenia, and 51 of their relatives; 29 with schizophreniform disorder, 30 of their relatives; 32 with major depression with psychotic features, 35 of their relatives; 33 with a bipolar disorder with psychotic features, 32 of their relatives), and 169 normal control subjects. Group comparisons demonstrated that (1) the probands with chronic schizophrenia, first episode schizophrenia, and schizophreniform disorder did not differ from one another on PVS; (2) these subjects combined had higher PVS ratings than the other two proband groups and normal subjects combined (who did not differ); and (3) none of the relative groups significantly differed from either one another or the normal subjects. On the other hand, relatives of schizophrenia spectrum probands with high PVS (greater than or equal to 10.0) had higher PVS ratings than the relatives of such probands with low PVS. Patterns of familial correlations suggested that PVS was moderately heritable (0.40). There was no evidence that nonadditive genetic variation influenced the trait.