HIV prevention programs for truck drivers and cleaners (TDC) in India are limited. Longitudinal follow-up presents an obstacle to program effectiveness evaluation. We asked 3,028 TDC in a truck-driver HIV prevention program in Hyderabad to leave a cellular telephone number; we contacted participants 6 months after the intervention to assess sexual risk behavior change. Married, older, and better educated participants were more likely to leave phone numbers. Only 6.5% of TDC were reachable after 6 months. Longitudinal follow-up of this mobile sub-population remains a challenge, and more effective methods for evaluating HIV prevention programs are needed.