Physicians' work with sickness certifications is an understudied field. The aims of this study were to gain knowledge of experiences concerning the sickness certification process among physicians working at oncology clinics. In 2008, all physicians working in Sweden (n = 36 898) were sent a questionnaire concerning sick-listing practices. All respondents working at an oncology clinic (n = 428) were included in the current study. Most of the physicians had sickness certification consultations at least weekly (91.3%). More than one fifth (22.3%) reported that they worked at a clinic with a workplace policy regarding the handling of sickness certification and 61.1% reported receiving at least some support in such cases from their immediate manager. Issuing unnecessary long sickness certificates were related to experiencing delicate interactions with patients and to lack of time. To a moderate degree, further competence was requested regarding: different types of compensation in the social insurance system, responsibilities of the Social Insurance Agency and employers, and sickness insurance rules. The large majority of physicians working in oncology reported regularly having consultations involving sickness certification. Overall, they reported few problems, low level of need for more competence regarding sickness certification, and low frequency of issuing sickness absences for longer periods than necessary.