The evolution of resistance of positive symptoms to antipsychotic therapy may represent a valuable means of subtyping schizophrenia. In contrast, resistance of negative symptoms and cognitive function to antipsychotic agents seems to be present from the first episode of psychotic symptoms and does not evolve over time to the same extent. If these findings are validated, this clearly points toward differences in the etiology of these components of schizophrenia. Data from a cohort of 223 patients with unsatisfactory responses to classical antipsychotic therapy are evaluated, at least 60% of whom responded to subsequent treatment with clozapine. Comparisons were made between the subgroups of patients with primary and delayed onset treatment resistance. Both subgroups responded to clozapine therapy, although better response was evident for patients with delayed resistance. The withdrawal of clozapine from patients who had previously been responsive to classical antipsychotic therapy was capable of inducing treatment resistance.