The BIA method, based on assessment of patients in activities, was developed to enable reliable assessment of clients' occupational functioning. The method evaluates the patient's ego functions and capacity for activity and participation. The aims of this study were to examine inter-rater reliability for the BIA method and to compare the experiences of staff working with the BIA with those of staff using standard assessment (SA). In SA, the patient's activity problems and capacities were assessed without using any structured protocol. Four staff members worked according to the BIA method and eight according to SA. The estimation of reliability was based on data from 14 patients who went through the BIA and were assessed by five staff members. These assessments resulted in all in about 400 statements, which were classified and compared for agreement between raters. In order to address the aim concerning staff experiences, a questionnaire was filled out anonymously by the staff. The inter-rater reliability of the nine ego functions varied from 0.76 to 1.00. The scale was deemed appropriate by the BIA staff and they had significantly higher median ratings on work satisfaction and appropriateness than the SA staff. In conclusion, the inter-rater reliability of the BIA was found to be good. The BIA method also seems to have a high degree of appropriateness, constituting a promising assessment tool when occupational functioning is addressed.