Thirty families were studied to determine genetic and evnironmental factors involved in holoprosencephaly. Those with chromosomal abnormalities were excluded. Many factors appear to cluster in proband families, such as mental retardation, mental illness, endocrine disorders, increased twinning and poverty level socioeconomic status. The empiric recurrence risk was 6%. Among 7 with lobar holoprosencephaly, there were 3 females and 4 males, while there were 19 females and 6 males with alobar holoprosencephaly.