Prophylactic swallowing exercises (PSE) during head-and-neck cancer (HNC) (chemo)radiotherapy (CRT) have a positive effect on swallowing function and muscle strength. Adherence rates to PSE are, however, moderate to low, undermining these effects. PRESTO already showed that the service-delivery mode (SDM), the way the exercises are offered, can influence adherence. The aim of this study was to investigate the effect of SDM on swallowing function and muscle strength during and post-CRT. In addition, the effect of overall adherence (OA), independent of SDM, was also investigated. A total of 148 HNC patients, treated with CRT, were randomly assigned to one of the three SDM's (paper-supported, app-supported, or therapist-supported PSE) and performed a 4-week PSE program. OA was calculated based on the percentage of completed exercises. Patients were divided into OA levels: the OA75+ and OA75- group performed respectively ≥ 75 and < 75% of the exercises. Swallowing function based on Mann Assessment of Swallowing Ability-Cancer (MASA-C), tongue and suprahyoid muscle strength during and up to 3 months after CRT were compared between the SDM's and OA levels. Linear Mixed-effects Models with post hoc pairwise testing and Bonferroni-Holm correction was used. No significant differences were found between the three SDMs. Significant time effects were found: MASA-C scores decreased and muscle strength increased significantly during CRT. By the end of CRT, the OA75+ showed significantly better swallowing function compared to OA75-. Muscle strength gain was significantly higher in the OA75+ group. SDM had no impact on swallowing function and muscle strength; however, significant effects were shown for OA level. Performing a high level of exercise repetitions is essential to benefit from PSE.Trial registration ISRCTN, ISRCTN98243550. Registered December 21, 2018-retrospectively registered, https://www.isrctn.com/ISRCTN98243550?q=gwen%20van%20nuffelen&filters=&sort=&offset=1&totalResults=2&page=1&pageSize=10&searchType=basic-search.