Aim: To compare the lung function parameters of poorly controlled type-1-diabetesmellitus (T1DM) patients with age-; height and sex-matched healthy-non-smokers (HNS).Population and methods: Subjects aged 35-60 Yrs who have a poorly controlled T1DM (glycated-Haemoglobin level >7%) with a disease history of more than 10 Yrs (n = 14) and HNS subjects (n = 14) were recruited. Clinical, anthropometric and fasting biological data were This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Aging; Case-control study collected. Plethysmographic data (flows, volumes, estimated-lung-age (ELA), lung-capacity-totransfer-carbon-monoxide (DLCO)) were measured. Large-airway-obstructive-ventilatory-defect (LAOVD) was defined as first-second-forced-expiratory-volume (FEV 1 )/forced-vital-capacity (FVC) below the lower-limit-of-normal (LLN). Restrictive-ventilatory-defect (RVD) was defined as total-lung-capacity (TLC) < LLN. Lung-hyperinflation was defined as residual-volume (RV) > upper-limit-of-normal. Student t-test and chi-2 test were used to compare plethysmographic data and profiles of the two groups.Results: The two groups were matched in chronological-lung-age (CLA) (respectively 47 ± 7 vs. 50 ± 8 Yrs) and sex (7 males and 7 females in each group) and height. Compared to the HNS group, the T1DM one had significantly lower FEV 1 , FVC, slow-vital-capacity and maximal-midexpiratory-flow (respectively 99 ± 11% vs. 83 ± 11%, 99 ± 9% vs. 86 ± 11%, 80 ± 8% vs. 67 ± 15% and 98 ± 23% vs. 72 ± 23%), had significantly higher TLC and RV (respectively, 105 ± 20% vs. 123 ± 24% and 108 ± 22% vs. 131 ± 24%) and had significantly higher percentage of subjects with lung-hyperinflation (7.1% vs. 43.0%). Both groups had similar percentages of LAOVD and RVD and similar corrected DLCO values. ELA of the T1DM group (57 ± 10 Yrs) was significantly higher than CLA.Conclusion: Poorly controlled T1DM seems to alter ventilatory mechanics without effect on the alveolo-capillary-membrane. In addition, it accelerates the respiratory ageing.