The objective of this study was to identify the factors that influence adherence to antiretroviral medication regimens in culturally diverse populations with human immunodeficiency virus/acquired immune deficiency syndrome (HIV/AIDS) related to age, ethnicity, and intravenous drug use. The critical incident technique was used to identify factors determining adherence. Through a series of brief, focused interviews, patients were asked to recall specific incidents that affected their adherence to HIV medications. Patients' responses were grouped into categories using Ethnograph software. Two nurse experts, with 94% agreement, determined the reliability of the category listings. A chi(2) analysis compared the proportion of patients reporting each factor across age, ethnicity, and intravenous drug use categories. The total sample of 52 participants included 46 male outpatients with HIV/AIDS receiving care through a Veteran's Administration Hospital in northern California and 6 health care providers. The interviews yielded a total of 294 incidents. Six major categories were identified, and a taxonomy of critical factors determining highly active antiretroviral therapies (HAART) adherence was developed. The categories included reminders and cues (30%), planning (18%), response to HIV/AIDS (17%), HIV medication characteristics (14%), interactions with others (12%), and patient characteristics (10%). The two largest categories representing patient-related factors accounted for nearly half of the incidents. Patients most influenced by positive support from others included non-Caucasians, those under the age of 50 years, and those with fewer years of education. The taxonomy of adherence behaviors provides important information for developing culturally relevant patient adherence education programs and an essential foundation for developing future studies.