To describe the determinants of delayed HIV presentation in one Northern California County, the authors identify persons with an opportunistic infection (OI) at HIV diagnosis. From 2000-2002, a sample of HIV patients attending a public AIDS program (n=391) were identified. Immigrants composed 24% of our sample; 78.7% of immigrants were Hispanic. Immigrants, compared to U.S.-born patients, presented with lower initial CD4+ counts at diagnosis than U.S.-born patients (287 cells/mm(3) vs. 333 cells/mm(3), p=0.143), were more likely to have an OI at HIV diagnosis (29.8% vs. 17.2%, p=0.009), and were more likely to be hospitalized at HIV diagnosis (20.2% vs. 12.5%, p=0.064). We found only immigrant status was significantly and independently associated with delayed presentation. Interviews with 20 newly HIV diagnosed Hispanic patients suggest lack of knowledge regarding HIV risk, social stigma, secrecy and symptom driven health seeking behavior all contribute to delayed clinical presentation. The main precipitants of HIV testing for immigrants were HIV/AIDS related symptoms and sexually transmitted infection (STI)/HIV diagnosis in a sexual partner. These results support augmentation of STI/HIV voluntary clinical testing and partner notification services along the Mexico-California migrant corridor.