In two series of psychiatric patients (numbering about 6,000 and 2,000 respectively), the mean age of the mothers at the time of the patients' birth was found to be very significantly above expectation from the general population, and this was so for each of the major diagnostic groups. In the second series, the age of the fathers was also found to be very significantly above that expected from a sample survey of the general population, and this was so for each diagnostic group. Fathers' age was raised more than mothers', and was highest for schizophrenia. The raised parental age could not be explained in terms of the patient's year of birth or his father's social class. The raised mothers' age could largely be accounted for by regression on the raised fathers' age. The present findings, and those of previous studies, seem best explained on the hypothesis of a constitutional parental trait leading to delayed marriage.