Background: Low back pain (LBP) is the most common diagnosis responsible for sick leave, long-term disability payments, and early retirements. Studies have suggested that the relatively small proportion of patients referred to a specialist for treatment, either conservative or surgical, accounts for most of the total costs of back pain. However, a complete and long-term picture of the socioeconomic burden associated with these two treatment regimens is lacking. Methods: From a cohort encompassing the entire population in Denmark (5.8 million inhabitants), we identified patients with LBP referred to specialised treatment, either conservative or surgical, during 2007–2016. According to treatment modality, two different cohorts were constructed. Each patient was matched with ten background population controls based on age, sex, region of residency and time of treatment (month and year). Using extensive, nationwide register data, the healthcare costs and loss of productivity from two years before the first intervention until 2018 was investigated. Findings: A total of 56,694 patients underwent surgical treatment, and 72,915 patients conservative treatment. Both cohorts had a significantly higher baseline cost two years before treatment compared with the background population controls. These measures increased sharply during the year after treatment. Five years after treatment, healthcare costs and loss of productivity remained proportionally similarly increased for the two treatment groups compared to the background population. Multiple surgeries had detrimental effects on long term productivity for the patients, and spouses to patients had marginally increased loss of productivity. Interpretation: The results show that patients referred to specialised treatment of LBP display poor socioeconomic prognosis, regardless of conservative or surgical treatment modality. This development was reinforced in patients undergoing multiple surgeries and was also observed among spouses to the patients. Our findings of substantial loss of productivity across subgroups indicate that measures of successful treatment need to be more nuanced.