Abstract Background Symptoms that cannot be attributed to any known conventionally defined disease are highly prevalent in general practice. Yet, only severe cases are captured by the current diagnostic classifications of medically unexplained symptoms (MUS). This study explores the clinical usefulness of a proposed new diagnostic category for mild-to-moderate conditions of MUS labelled ‘multiple symptoms’. Methods A mixed methods approach was used. For two weeks, 20 general practitioners (GPs) classified symptoms presented in consecutive consultations according to the International Classification of Primary Care (ICPC) supplemented with the new diagnostic category ‘multiple symptoms’. The GPs’ experiences were subsequently explored by focus group interviews. Interview data were analysed according to ethnographic principles. Results In 33% of patients, GPs classified symptoms as medically unexplained, but applied the category of ‘multiple symptoms’ only in 2.8%. The category was described as a useful tool for promoting communication and creating better awareness of patients with MUS; as such, the category was perceived to reduce the risk of unnecessary tests and referrals of these patients. Three main themes were found to affect the clinical usefulness of the diagnostic category of ‘multiple symptoms’: 1) lack of consensus on categorisation practices, 2) high complexity of patient cases and 3) relational continuity (i.e. continuity in the doctor-patient relationship over time). The first two were seen as barriers to usefulness, the latter as a prerequisite for application. The GPs’ diagnostic classifications were found to be informed by the GPs’ subjective pre-formed concepts of patients with MUS, which reflected more severe conditions than actually intended by the new category of ‘multiple symptoms’. Conclusions The study demonstrated possible clinical benefits of the category of ‘multiple symptoms’, such as GPs’ increased awareness and informational continuity in partnership practices. The use of the category was challenged by the GPs’ conceptual understanding of MUS and was applied only to a minority of patients. The study demonstrates a need for addressing these issues if sub-threshold categories for MUS are to be applied in routine care. The category of ‘multiple symptoms’ may profitably be used in the future as a risk indicator rather than a diagnostic category.