Abstract Context The long-term natural history of diabetic ketoacidosis (DKA) and its risk factors are poorly understood. Objective To determine the long-term incidence and predictors of DKA in adults with longstanding type 1 diabetes (T1D). Design All hospitalizations and deaths due to DKA between 1996 and 2016 were identified in 4758 adults with T1D from the Finnish Diabetic Nephropathy Study (FinnDiane), and a cohort of 16 224 adults with T1D from the Finnish general population. Results Between 1996 and 2015, there were 1228 DKA events in the FinnDiane participants (1.4/100 person-years) and 4914 DKA events (1.8/100 person-years) in adults with T1D from the general population. The majority were hospitalized only once. There was a modest increase in the frequency of DKA in the FinnDiane over the follow-up (~2.4%/year [95% CI, 0.3–4.5%]; P = 0.03). Predictors of DKA were glucose control, CSII, smoking and alcohol consumption, and raised high-density lipoprotein cholesterol and triacylglycerides. Diabetic nephropathy and renal impairment were associated with DKA; patients with end-stage renal disease, macroalbuminuria, and microalbuminuria had 2.09-fol (95% CI, 1.40–3.12), 1.65-fold (95% CI, 1.23–2.19), and 0.87-fold (95% CI, 0.61–1.24) risk of DKA compared with patients with normal albumin excretion rate, respectively. Patients with an estimated glomerular filtration rate <60 mL/min/1.73 m2 were also more likely to be hospitalized for DKA (HR 1.71 [95% CI, 1.26–2.67]). Conclusions DKA remains a common cause of hospitalization in individuals with longstanding T1D. These data suggest that the goal to use SGLT2 inhibitors for their vasculo- and renoprotective actions may be problematic, as those most likely to benefit may also have the highest risk for DKA.