OBJECTIVES: First, to investigate comorbidities associated with Alzheimer's disease (AD) and second, to report eligibility and the yearly trend of incidence rate of AD in Medi-Cal from 1995 to 2002. METHODS: A retrospective case-control study was conducted using 20% sample of Medi-Cal administrative claims data from 1995 to 2002. The cases were patients with a diagnosis of AD, and were 1:10 matched to a cohort without AD based on age and gender. Eligibility was measured by the number of eligible months. Relative risk (RR) was calculated to estimate the risk of AD patients developing a profile of 24 common comorbidities set forth by AHRQ. RESULTS: In total, 6,494 cases and 64,940 controls were identified. The average age was 84 and 69% were female. During the eight-year study period, both groups had around 50 eligible months with no significant difference. Hispanics and other minorities (primarily Asians) had lower risk of having AD (p<0.0001). In addition to depression (RR=1.67) and psychoses (RR=2.63), which are the known comorbidities of AD, peripheral vascular disorder (RR=1.49) and other neurological disorders (RR=1.82) were associated with the AD diagnosis. Interestingly, AD patients had significantly lower risk for many other chronic diseases such as CHF (RR=0.82), valvular disease (RR=0.85), hypertension (RR=0.84), COPD (RR=0.85), diabetes (RR=0.85), renal failure (RR=0.86), peptic ulcer (RR=0.86), cancer (RR=0.69), and rheumatoid arthritis (RR=0.80). The yearly incidence rate for AD slightly declined from 5.8 per 10,000 person-years in 1995 to 4.8 per 10,000 person-years in 2002. Average annual incidence rate was 5.3 per 10,000 person-years. CONCLUSIONS: The present study demonstrated that patients with AD had increased risk of some diseases, yet were less likely to experience many chronic disorders common to elderly patients.