The authors describe 12 patients with antipsychotic-induced hyperprolactinemia. Six patients had erroneous ideas of being pregnant (four delusional and two non-delusional) temporally associated with hyperprolactinemia and resolving as prolactin levels returned to normal. The remaining six patients did not develop such ideas. Contrasting the clinical features of the two groups of patients in the context of existing literature informs on the possible biological and cognitive mechanisms that can be hypothesized to underlie the relationship between hyperprolactinemia due to antipsychotics and the development of inaccurate beliefs and feelings about pregnancy, and the effect of current mental state on the propensity to develop these beliefs.