In the Netherlands, all procedures in general surgery are categorized into 12 surgery groups by the Association of Surgeons of the Netherlands. The purpose of this study was to assess whether surgery groups differ in adverse outcome probabilities, to decide whether hospital comparisons on adverse outcomes should be adjusted for differences in surgery groups. All surgical patients in one hospital discharged in 1997-1999 were included. Only the first operation during admission was included, with the assumption that successive operations were treatment of adverse outcomes. To avoid bias, only operations with procedures from the same surgery group were included. A total of 6,025 admissions were included and analyzed by a two-step multilevel analysis. Four surgery groups had fewer admissions with adverse outcomes than expected, and two groups had more. After adjustment for patient and operation characteristics, the remaining variance between surgery groups is still large. Similar results were found when differences in mortality were analyzed. Surgery group can therefore be used to adjust hospital comparisons for differences in surgical procedure mix.@en