Large individual differences exist among psychotic and retarded children, and a procedure that would enable classification of discrimination learning ability would be of value. A procedure designed to assess the discrimination learning thresholds of low-functioning children is described. A performance index (PI) that reflects accuracy of discriminative responding, difficulty of the discriminations attained, and learning rate was found to correlate significantly with mental age, intelligence quotient, Vineland Social Age, Vineland Social Quotient, and language functioning, but not with chronological age. The question of the optimal magnitude of correlation between a new measure and existing measures is discussed.