Vitamin D plays an important role in calcium homeostasis and is essential for normal bone growth and remodeling. Previous studies of acutely ill and institutionalized older women in Galway revealed that vitamin D insufficiency is extremely common. We aimed to evaluate age and gender-related rates of vitamin D insufficiency and deficiency in adult patients attending their General Practitioners in the same region.A retrospective cohort study.General practices in Galway, a city located on the west coast of Ireland, latitude 53.17 North.A total of 15,708 consecutive blood samples from adults attending their General Practitioners between January 2013 and December 2014 were sent to the Clinical Biochemistry Laboratory in the University Hospital Galway for vitamin D level testing. 73% were from women.Data were collected and grouped according to age, sex and season. Vitamin D levels (25OHD) were categorised into deficiency (< 25nmol/L), insufficiency (25-50nmol/L) and sufficiency (> 50nmol/L).Median and interquartile range were reported as vitamin D levels were not normally distributed. Descriptive statistics were performed using STATA/MP 13.1 for Mac.While vitamin D levels were sufficient in 56.4% (n=8,496), they were insufficient in 32.4% (n=4,891) and deficient in 11.2% (n=1,691). Of the deficient group, 19%(325) had levels < 15nmol/L consistent with profound deficiency. Men were more likely than women to be deficient (12% (n=489) vs. 10.9% (n=1,202)) and insufficient (38.0% (n=1,550) vs. 30.4% (n=3,341)) (p < 0.01). Highest rates of deficiency were seen in those 81-97 years (19.4% (n=136)) and 18-40 years (13.8% (n=543)), the former group showing less seasonal variation.Overall prevalence of insufficiency is similar to those in national and European studies published recently. However, there is a high possibility that those most vulnerable to vitamin D deficiency were underrepresented in this cohort, when results from previous studies in the same region are considered. Further prospective studies are needed to guide treatment with supplementation of vitamin D for appropriate subgroups.