Background: Base-line data from a community-based HIV outreach effort serving Puerto Rican injection drug users (IDUs) in Massachusetts identified that approximately half of their clients were born on the mainland and half on the island. Methods: Logistic regression methods examined the relationship between place of birth, primary language spoken, primary residence of family and needle sharing for a sample of 200 Puerto Rican IDUs residing in Massachusetts. Focus groups were used to interpret quantitative findings. Results: A logistic regression model indicated that Puerto Rican IDUs born on mainland USA were 2.1 times more likely to share needles than IDUs born in Puerto Rico, after controlling for gender, age, education, drug overdose, incarceration history and psychiatric status. Also, Puerto Rican IDUs who were older had overdosed on drugs in the past year, had been incarcerated in their lifetime, and were homeless were significantly more likely to report having shared needles in the past 6 months compared to their counterparts. Focus group interviews with Puerto Rican outreach workers and individuals in recovery suggested that differences in needle sharing by mainland versus island born IDUs may be due to cultural differences in interpretation of the interview questions. Implications: Researchers examining HIV risk behaviors among culturally diverse substance abusers need to conduct more mixed-method studies to identify if different cultural groups understand quantitative measures differently. Incarceration may be a significant risk factor in the continued spread of HIV among IDUs and expanded HIV prevention efforts need to be developed that specifically target this high-risk group. (c) 2007 Elsevier Ltd. All rights reserved.