OBJECTIVE: To assess avoidable hospitalization as an outcome indicator of access to and quality of primary care by examining trends and gender and ethnic variations. METHOD: Aggregated nationwide data on annual hospital discharges in Singapore from 1991 to1998 were collected from the Central Claims Processing System (CCPS) database. They included total hospitalizations (excluding maternity and psychiatric disorders) and hospitalizations for chronic diseases, which are avoidable by timely, appropriate and effective primary care (ambulatory care sensitive (ACS) conditions): asthma, congestive heart failure, chronic obstructive pulmonary disease, diabetes mellitus and hypertension. Directly age-standardized rates were adjusted for gender and ethnic differences in total hospitalizations. RESULTS: From 1991 to 1998, (total of 1,479,494 hospitalizations), 6.7% were for ACS conditions. Overall, the annual rate of avoidable hospitalization was 29.4 per 10,000 population. Women had lower rates of avoidable hospitalizations than males (22.4 versus 29.5 per 10,000), as well as for total hospitalizations (496.2 versus 515.5 per 10,000). Adjusted for total hospitalization, males were 1.3 times more likely than females to be hospitalized for ACS conditions. Compared to Chinese, Indian and Malays had higher rates of avoidable hospitalizations (21.7, 65.5 and 56.1 per 10,000 respectively). Adjusting for their higher rates of total hospitalization, they were 1.7 and 1.8 times respectively more likely than Chinese to be admitted for ACS conditions. Avoidable hospitalization decline (adjusted for total hospitalization) was -9.1% overall; greater in males (-11.8%) than in females (-5.3%); greater for Chinese (-15.8%), than Malays (-1.1%) and Indians (increase of +4.3%). Among Malays and Indians, women showed marked increases of avoidable hospitalizations, +10.8% and +8.1% respectively. CONCLUSION: We demonstrated gender and ethnic differences and temporal trends in avoidable hospitalization. Avoidable hospitalization is a sensitive tool for monitoring access to and adequacy of primary ambulatory care.