||BACKGROUND: Suggested strategies in reducing the impact of non-communicable diseases (NCD) are early diagnosing and screening. We have limited proof of benefit of population screening for NCD. Increased mortality in persons with diagnosed NCD has been shown for decades. However, mortality in undetected NCD has barely been studied. This paper explores whether all-cause mortality differed between persons with diagnosed hypothyroidism, type 2 diabetes (T2DM), and hypertension, compared with persons with undetected-, and with persons without the corresponding disease. METHODS: A prospective cohort study of the general population in Nord-Trondelag, Norway. Persons >/=20 years at baseline 1995-97 were followed until death or June 15, 2016. Cox proportional hazards models were used to compute age and multiple adjusted hazard ratios (HR) with 95% confidence intervals (CI) for the association between disease status and all-cause mortality. The number of participants in the hypothyroidism study was 31,960, in the T2DM study 37,957, and in the hypertension study 63,371. RESULTS: Mortality was increased in persons with diagnosed type 2 diabetes and hypertension, compared to persons without corresponding disease; HR 1.69 (95% CI 1.55-1.84) and HR 1.23 (95% CI 1.09-1.39), respectively. Among persons with undetected T2DM, the HR was 1.21 (95% CI 1.08-1.37), whilst among undetected hypothyroidism and hypertension, mortality was not increased compared with persons without the diseases. Further, the association with mortality was stronger in persons with long duration of T2DM (HR 1.96 (95% CI 1.57-2.44)) and hypertension (HR 1.32 (95% CI 1.17-1.49)), compared with persons with short duration (HR 1.29 (1.09-1.53) and HR 1.16 (1.03-1-30) respectively). CONCLUSIONS: Mortality was increased in persons with diagnosed T2DM and hypertension, and in undetected T2DM, compared with persons without the diseases. The strength of the association with mortality in undetected T2DM was however lower compared with persons with diagnosed T2DM, and mortality was not increased in persons with undetected hypothyroidism and hypertension, compared with persons without the diseases. Thus, future research needs to test more thoroughly if early diagnosing of these diseases, such as general population screening, is beneficial for health.