Adolescence is associated with an increase in the rate of certain psychiatric symptoms, and it is typically the developmental period when prodromal features of the major psychiatric disorders emerge. This is especially true of schizophrenia, with the majority of patients showing a marked postpubertal rise in schizotypal signs that predates the onset of clinical symptoms in early adulthood. Cross-sectional studies of youth have revealed a positive correlation between age and saliva cortisol level, suggesting a normative maturational increase in activity of the hypothalamic–pituitary–adrenal (HPA) axis. It has been hypothesized that this increase may trigger the expression of symptoms in vulnerable individuals. The present longitudinal study measured cortisol secretion and its relation with symptom development in samples of youth with schizotypal personality disorder (SPD), other personality disorders, or no Axis II disorder. The findings indicate moderate stability in cortisol levels across a 2-year period, with a longitudinal increase in cortisol levels over time. Cortisol levels at the first and second assessments were correlated with the severity of SPD symptoms at follow-up. The results are consistent with the notion that the HPA axis undergoes a postpubertal maturational process that moderates the expression of psychiatric symptoms.