Background. Intervention fidelity is an increasingly important methodological concept in process evaluations. In this article, the authors investigated the intervention fidelity in a randomized controlled trial on excessive weight gain prevention in pregnancy. Method. A sample of 109 audiotaped counseling sessions, linked to 65 women in the intervention group of the New Life(style) trial, was drawn. The following criteria were quantitatively evaluated using a fidelity checklist: (a) reach, (b) dose, (c) adherence to study objectives, (d) adherence to underlying problem-solving treatment (PST) theory, and (e) counselor competence. Results. A total of 60.4% received all counseling sessions. The dose of intervention components was generally moderate (50.9% to 60.4%), and the dose of PST components was low (17.3%). Adherence to study objectives was moderate (64.2%) and adherence to PST theory was low (43.2%). The counselors sufficiently stimulated the participant to optimize lifestyle (54.2% of the sessions), provided positive feedback (50.5%), and left the initiative regarding problem solving to the participant (71%). One of the two counselors performed significantly better on all measured criteria (p<.001). Conclusions. Intervention fidelity in the New Life(style) trial was generally low to moderate. In future interventions, it is recommended to put more emphasis on counselor recruitment, training, and intervention protocol contents. Fellow researchers are encouraged to embed a process evaluation into all study stages, taking into account all essential process elements, and to link process outcomes to more distal, health outcomes. © 2012 Society for Public Health Education.