There is considerable variability in the severity of bipolar disorder, e.g., in terms of the frequency of inpatient episodes. The long-term progression also differs, where some patients are sensitised with progressively shorter healthy intervals. Little is known about the proportion of patients being sensitised, their clinical characteristics, and biological underpinnings. We analysed long-term progression of bipolar disorder in relation to clinical characteristics (N?=?3074), serum biomarkers (N?=?745), and genetic variants (N?=?1401) in a cohort of Swedish bipolar disorder patients. We took advantage of the National Patient Register, providing reliable data on 35,973 psychiatric inpatient care episodes in Sweden since 1973. First, one third of the cohort cluster together with a maximum of one inpatient episode per year, while the remaining two thirds had >1 episode per year. These groups did not differ with respect to clinical features or biomarkers. Second, among patients with at least five inpatient episodes (defined as severely ill), we find one group with progressively shorter cycle-lengths (one fifth of the total cohort, N?=?550). Compared with those with a stable or recuperant trajectory, these patients featured lower functioning, more antidepressant treatment, as well as reduced levels of inflammatory markers in serum. Third, sensitisation was associated with a common genetic variant near the calcium channel gene CACNA2D3 at genome-wide significance. These results suggest the potential for translational research aimed at preventive actions.