Obsessive Compulsive Disorder - Melvin actually undergoes therapy for this. The Obsessive Compulsive Disorder, or OCD, is an anxiety disorder in which the individual demonstrates an inability to contain and control interfering thoughts and compulsive actions. Melvin repeatedly checked all three locks on his door every time he would come into his apartment and lock the door and before putting on his slippers he would tap the sides three times. Another characteristic of OCD is a fear of contamination or germ phobia. In one of the opening scenes of As Good As It Gets, Melvin is seen washing his hands in what is obviously boiling hot water then cleansing with one soap and dispensing it, then rinsing again in the hot water and then washing again with another bar. Also, Melvin never uses the silverware at the restaurant...he always brings his own plastic ware. Predominant characteristics of OCD are anxiety and repetitive behaviors. The direct cause for OCD has yet to be determined however, an imbalance of serotonin, a monoamine neurotransmitter important in controlling emotional states and tendencies such as impulses. A low level of serotonin can result in an anxious mood. Brain scans have shown that brain activity within a brain afflicted by OCD definitely has a different activity pattern than that of a normal brain.

Anal-Retentive Personality - Anal retentiveness can be defined in two ways; a preoccupation with bowels and a preoccupation with small details. Melvin has both. In fact, the very first scene in the film is Melvin trying to get Verdell out of the apartment building so that he may do his doggy business elsewhere. Seeing as how Verdell is not his dog, where Verdell defecates should not concern Melvin. But it does. It does so much that he is willing to put cute little Verdell into the garbage shoot. Furthermore, Melvin continually shows his preoccupation with small, unimportant things such as the goings on of his neighbors and the process of packing for a weekend trip.