Background: Parents’ perspectives on their children’s treatment process and outcomes are valuable to treatment development and improvement. Participants’ engagement in Routine Outcome Monitoring (ROM) has, however, been difficult and may particularly be so in specialized settings, such as semi-residential psychiatry. In this paper, the use of a web-based ROM system implemented in a child semi-residential psychiatric setting is described and predictors associated with low completion rates of questionnaires by parents are identified. Methods: Parents and the multidisciplinary team of 46 children admitted to semi-residential psychiatric treatment participated in this study and completed a battery of questionnaires in three month intervals. Results: The overall completion rate of both parents during ROM assessment was 77 % compared to 83 % of all clinicians involved. Completion of questionnaires by parents was higher around first assessments and declined after a year treatment. For eight clients at least one of the parents stopped filling out questionnaires during ROM measuring. Logistic multilevel analyses revealed initial treatment factors associated with a low completion of questionnaires by parents during ROM: high comorbidity of the child on DSM Axis I, single parenthood, a higher parental educational level and having a weaker therapeutic alliance regarding goal setting. Conclusions: The findings in this paper demonstrate relatively high completion of questionnaires by clinicians and parents when using ROM in child semi-residential psychiatry. Strong administrative and electronic support undoubtedly contributed to this result. Clinicians are encouraged to motivate parents to mutually invest in ROM, and to take into account factors indicating a possible lower completion of questionnaires by parents.