An extensive analysis of prevalence rates of cognitive impairment and other mental morbidities was carried out as part of a five-site national study on the health and mental health of an ambulatory population. This study reports on prevalence rates contrasted by age across the 18 and over population for cognitive impairment and other diagnoses in the Baltimore, Maryland, site of this study. Differences in prevalence rates by age are striking. Eight conditions have rates above 1 per cent among those 64 and younger: phobia (13.8 per cent), alcohol use disorder (6.5 per cent), obsessive compulsive disorder (2.2 per cent), schizophrenia (1.4 per cent), and panic disorder (1.2 per cent). For the older group, 65 to 74 years, five conditions have such prevalence rates: phobic disorder (12.1 per cent), severe cognitive impairment (3.0 per cent), alcohol use disorder (2.1 per cent), obsessive compulsive disorder (2.2 per cent), and dysthymia (1.0 per cent). For the oldest group, those 75 and over, only four conditions have rates of 1 per cent or more. These are: phobic disorders (10.1 per cent), severe cognitive impairment (9.3 per cent), major depression (1.3 per cent), and dysthymia (1.1 per cent). Rates of cognitive impairment increase markedly with age and high rates of this disorder were found among those never married, separated, divorced, or widowed. Implications of these findings for understanding mental morbidity among the elderly and issues for future planning are discussed.