The range of cognitive gains made by low-income preschool children in the home-based Mother-Child Home Program is discussed as to the causes of the wide variability found. At the end of one year (October 1967 to May 1968) in the program, 33 low-income preschoolers made an average Stanford-Binet IQ gain of 17 points. The varibility within this group ranged from a gain of 33 points to a loss of 7 points. The average IQ gain for 26 similar children new to the program in the following year (October 1968 to May 1969) was approximately 11 points, with a variability ranging from a gain of 24 points to a loss of 4 points. The first group was divided, at the 17 mean point gain in IQ, into High Gainers and Low Gainers (from -7 to 8 points). The seven children who were identified as Low Gainers evidenced a common pattern of verbally related behavior within the intervention itself, which appears to be characterized by social and cognitive immaturity and by a relatively frequent negative affective tone; six of the seven were also related by an indication of unhappiness in family relationships. It is felt that the factors, beside the program itself, which are associated with high and low cognitive gain, should be identified. Nine case studies of children in the first year of the program are given as illustrations. (DB)