Issues surrounding sickness absence are of interest due to growing awareness of the costs to employers and the UK economy, a greater understanding of the interaction between health and work, and increasing evidence that work is beneficial to physical and mental well-being. The Health & Occupation Reporting network in General Practice (THOR-GP) is a national source of information on work-related sickness absence.To assess the factors influencing work-related sickness absence in the UK.General practitioners (GPs) report cases of work-related ill-health via an online web form. Sickness absence information reported with each case was compared by demographic information, diagnosis/symptom and employment factors.Between 2006 and 2009, THOR-GP received 5683 case reports of work-related ill-health; 53% were musculoskeletal diagnoses and 31% were mental ill-health diagnoses. Over half (56%) of cases reported had associated sickness absence. Diagnosis had a highly significant influence on the occurrence of any associated sickness absence. Eighty-one per cent of mental ill-health cases were reported to result in sickness absence compared to 50% of musculoskeletal cases. Public sector employees incurred sickness absence more frequently than those from the private sector. Industries with the highest mental ill-health incidence rates had sickness absence episodes most frequently. Within employment groups, levels of sickness absence were inversely proportional to the level of self-employment.These data reported by GPs with vocational training in occupational medicine may help to inform policy decisions targeting work-related exposures and the management of sickness absence, thereby reducing the UK burden of work-related sickness absence.