An attempt has been made to contribute to the understanding of the symptoms and factors associated with the Blastocystis Hominis infection, as seen in persons seeking outside consultation from the Dermatological and Transmissible Diseases Department (DTDD) at the C.H.N.H. This is a case-control study carried out in people between the ages of 5 and 80 in a period from January to March 1999. The cases tested positive in parasitological tests for Blastocystis Hominis and were absent of other enteropathogens. The controls tested negative in parasitological tests for Blastocystis Hominis and were absent of other enteropatoghens. A clinical chart was used to register details of symptomatology and factors associated with the Blastocystis Hominis infection. 74 cases and 70 controls were studied, matched by sex and age. A statistical correlation was obtained (p<0.05) among symptomatic persons and presence of Blastocystis Hominis (91,9%). The symptomatology associated with the Blastocystis Hominis infection by order of statistical significance (p<0.05) was: Abdominal pain (OR=3) 1.47<OR<6.60, abdominal ballooning (OR=2.36) 1.06<OR<5.29, urticaria (OR=3.19) 0.81<OR<12.48. The only risk factor associated with the Blastocystis Hominis infection was the consumption of unboiled water (OR=2.52) 1.01<OR<5.83. In conclusion, Blastocystis Hominis is associated to symptomatic subjects with abdominal pain and ballooning and urticaria, who possess at least two or three positive tests. This infection would be facilitated by the consumption of unboiled water.