Adults with Down syndrome (n = 160) and their aging mothers were compared with adults who have mental retardation due to other causes (n = 253) and their mothers in order to determine the nature and direction of diagnostic group differences at the middle and later stages of the family life course. Consistent with research on young families with a child who has Down syndrome, aging mothers of adults with Down syndrome reported less conflicted family environments, more satisfaction with their social supports, and less stress and burden associated with caregiving than did mothers whose adult child's mental retardation was due to other causes. Factors associated with patterns of stability and change in diagnostic group differences were discussed.