ObjectivesMultimorbidity, the co-occurrence of multiple long-term conditions, is common and increasing. Definitions and assessment methods vary, yielding differences in estimates of prevalence and multimorbidity severity. Sociodemographic characteristics are associated with complicating factors of multimorbidity. We aimed to investigate the prevalence of complex multimorbidity by sex and occupational groups throughout adulthood.DesignCross-sectional study.SettingThe third total county survey of The Nord-Trøndelag Health Study (HUNT), 2006–2008, Norway.ParticipantsIndividuals aged 25–100 years with classifiable occupational data and complete questionnaires and measurements.Outcome measureComplex multimorbidity defined as ‘the co-occurrence of three or more chronic conditions affecting three or more different body (organ) systems within one person without defining an index chronic condition’.AnalysisLogistic regression models with age and occupational group were specified for each sex separately.Results38 027 of 41 193 adults (55% women) were included in our analyses. 54% of the participants were identified as having complex multimorbidity. Prevalence differences in percentage points (pp) of those in the low occupational group (vs the high occupational group (reference)) were 19 (95% CI, 16 to 21) pp in women and 10 (8 to 13) pp in men at 30 years; 12 (10 to 14) pp in women and 13 (11 to 15) pp in men at 55 years; and 2 (−1 to 4) pp in women and 7 (4 to 10) pp in men at 75 years.ConclusionComplex multimorbidity is common from early adulthood, and social inequalities persist until 75 years in women and 90 years in men in the general population. These findings have policy implications for public health as well as healthcare, organisation, treatment, education and research, as complex multimorbidity breaks with the specialised, fragmented paradigm dominating medicine today.