A major problem facing clinical blood banks is the procurement of numbers of altruistic blood donors sufficient to guarantee the quality of blood components. Adverse reactions to donation can negatively influence altruistic donors for new donations. A cross sectional study was carried out at Santa María Cardiovascular Clinic in Medellín during 1999 to identify the frequency and risk factors involved in adverse reactions to donation. The donors mean age was 33 years in females and 34 in males.The frequency of adverse reactions was 2.7%; the most important class was vasovagal reactions in 1.2%. Risk factors related to adverse reactions were first-time donations, age below 25 years and femaleness.