1. Eighty psychotic children studied at Oxford or Newcastle have been classified as suffering from infantile psychosis (I.P.) if the illness began before the age of 3, or from late onset psychosis (L.O.P.) if it began later than 5. 2. This division was found to correspond to a distinction in clinical phenomena. The L.O.P. group displayed disorder of form and stream of thought and disordered thought content, were commonly hallucinated (especially auditorily), auditory and showed perplexity or an attitude of suffering. The I.P. group showed none of these but was characterized by severe speech delay and many speech anomalies, especially echolalia, by unusual responses to noises, by indifference to pain, by overactivity, by stereotyped movements, by self-directed aggression and a spread of compulsive acts, and by poor relationships with others especially as shown by gaze avoidance. 3. The problems of accurately assessing the presence of these and other clinical phenomena are discussed. 4. By ranking the symptoms in each group according to their frequency of occurrence it is made clear which particular symptoms are diagnostically discriminant. 5. L.O.P. cases with a simple schizophrenic picture may be confused with I.P. cases of the same age if a clear history is not available, but cases with pronounced symptoms are easily distinguishable.