Insomnia co-occurs with many health problems, but less is known about the prospective associations. The aim of the current study was to investigate if insomnia predicts cumulative incidence of mental and physical conditions. Prospective population-based data from the two last Nord-Trøndelag Health Studies (HUNT2 in 1995–97 and HUNT3 in 2006–08), comprising 24 715 people in the working population, were used to study insomnia as a risk factor for incidence of physical and mental conditions. Insomnia was defined according to the 4th edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV). Insomnia at HUNT2 was a significant risk factor for incidence of a range of both mental and physical conditions at HUNT3 11 years later. Most effects were only slightly attenuated when adjusting for confounding factors, and insomnia remained a significant risk factor for the following conditions in the adjusted analyses: depression [odds ratio (OR): 2.38, 95% confidence interval (CI): 1.91–2.98], anxiety (OR: 2.08, 95% CI: 1.63–2.64), fibromyalgia (OR: 2.05, 95% CI: 1.51–2.79), rheumatoidarthritis (OR: 1.87, 95% CI: 1.29–2.52), whiplash (OR: 1.71, 95% CI: 1.21–2.41), arthrosis (OR: 1.68, 95% CI: 1.43–1.98), osteoporosis (OR:1.52, 95% CI: 1.14–2.01, headache (OR: 1.50, 95% CI: 1.16–1.95,asthma (OR: 1.47, 95% CI: 1.16–1.86 and myocardial infarction (OR:1.46, 95% CI: 1.06–2.00). Insomnia was also associated significantly with incidence of angina, hypertension, obesity and stroke in the crude analyses, but not after adjusting for confounders. We conclude that insomnia predicts cumulative incidence of several physical and mental conditions. These results may have important clinical implications, and whether or not treatment of insomnia would have a preventive value for both physical and mental conditions should be studied further.