A quasi-experimental study was conducted at a Dutch sexually transmitted infection (STI) clinic to compare the effects of educational counseling and motivational interviewing (MI)-based HIV/STI counseling on determinants of condom use and partner notification at 6-month follow-up. It also examined the feasibility of MI-based counseling in a busy real-life clinic. The counseling approaches were historically compared: respondents in the control condition were recruited between April and July 2005, those in the experimental condition between September and December 2005. The study involved 428 participants. These were all high-risk clients of the STI clinic. Their mean age was 33.7 years, and 39.6% were female. The study showed that MI-based counseling had a more positive effect on self-efficacy, intentions to use condoms with casual partners, and long-term condom use with steady partners. It had no adversarial outcomes on other social cognitions or behaviors compared to educational counseling. Furthermore, MI-based counseling is experienced as a more respectful and structured way of counseling. MI-based counseling was relatively easily implemented into the current clinic procedures. In addition to the implementation of the training, neither specialized staff nor additional or longer client visits were needed. However, some nurses indicated that the new method required more personal investment and effort. Limitations of the current study are the low response rates, the high educational level of most participants, and the small sample size regarding partner notification. Nonetheless, we conclude that MI-based counseling was a more effective approach to preventive counseling compared to educational counseling and feasible in the busy real-life setting.