['To study if vitamin D fortification of milk products started in February 2003 has improved vitamin D status of young Finnish men, which has been poor before.', 'A longitudinal study of one cohort.', 'Helsinki University Central Hospital.', 'Sixty-five healthy men, studied for the first time in January 2001, were re-examined in January 2004. They were aged 18-21 years in 2001.', 'Blood was sampled for determination of serum 25-hydroxyvitamin D (25-OHD) and intact parathyroid hormone (iPTH). 25-OHD was measured by both radioimmunoassay (RIA) and high-pressure liquid chromatography (HPLC). Consumption of milk, sour milk and fish and use of vitamin D supplements were assessed using a questionnaire.', 'In January 2004, vitamin D fortification had raised serum 25-OHD level, with the mean of individual percent changes being 20.4% measured with RIA (P=0.0015). The correlation between the RIA and HPLC methods was high (r=0.85). Nineteen men (29.2%) had vitamin D deficiency (25-OHD<or=20 nmol/l) and 48 men (73.8%) had hypovitaminosis D (25-OHD<or=37.5 nmol/l). Serum 25-OHD and iPTH levels correlated inversely (r=-0.30; P=0.017). Serum 25-OHD levels were 52.0% higher for 13 vitamin D supplement users than for 51 non-users (P=0.0001). After exclusion of supplement users, an association between 25-OHD level and consumption of milk products was found (P=0.011), with the median vitamin D level being 6.5 nmol/l higher for those consuming four glasses of milk products or more daily than for those consuming one or less. Likewise, the higher the milk consumption, the higher the difference in 25-OHD between years 2004 and 2001 (P=0.0025).'] Does vitamin D fortification of milk products resolve hypovitaminosis D in young Finnish men?