We studied the conservation scores of 21 schizophrenic and schizotypal children (aged 6;7–12;5 years) and 21 yoked normal mental age matches and their relationship to the Kiddie Formal Thought Disorder Scale measures. Like the normal children, the schizophrenic/schizotypal subjects were able to recognize the invariance of two dimensional space, number, and substance, but not of weight. Unlike the normal children, the schizophrenic/schizotypal children were poor conservers of continuous and discontinuous matter, and this was related to their illogical thinking scores. Age and IQ accounted for the conservation competence of the normal children. The conservation skills of the patients, however, were associated with the severity of their formal thought disorder scores, not with their age or IQ. The developmental approach employed in this study enabled further clarification of the cognitive parameters of formal thought disorder in middle childhood. We discuss the developmental, clinical, and possible information processing implications of these findings.