PURPOSE: To assess the seroprevalence of IgG and IgM specific for parvovirus B19 (B19) and its association with aplastic crisis developing in patients with different haematological disorders. PATIENTS AND METHODS: Fifty-three serum samples were evaluated, 24 from patients with aplastic crisis and 29 from others without such crises, all of them suffering from different haematological diseases diagnosed at the University Hospital of Maracaibo and the Zulia State Blood Bank, in Venezuela; 15 samples from healthy blood donors were examined as well. Indirect immunofluorescence technique was used in the study. Lymphocyte subsets were quantified in 10 of the patients with aplastic crisis by means of cytofluorometry. Serum proteins were assessed by electrophoresis in all samples. The statistical analysis was performed according to Student's t test and chi square, by applying the statix 4.0 and SAS programmes. RESULTS: Positive IgG was found in 20 of the 24 patients with aplastic crisis (83.3%), 20 of the 29 without crisis (68.9%) and 7 of the 15 healthy controls (46.6%). Positive IgM was found only in 2 of the 24 patients with aplastic crisis (8.3%). Both the patients without aplastic crises and the control groups were negative for PB19 IgM. The average CD4 and CD8 T lymphocyte count and the CD4-CD8 index in the patients studied were 454 CD4 cells/microL, 1,006 CD8 cells/microL and 0.5%, significantly different from the control group, whose figures were 860 CD4 cells/microL, 546 CD8 cells/microL and 1.6%. The average B lymphocyte count of the patients (628 cells/microL) was higher than that of the control group (349 cells/microL). The average NK cell count in the patient was 174 cells/microL, slightly below the control group (221 cells/microL). Mild beta-globulin decrease was found in the electrophoretic study of the serum proteins of the patients, along with significant increase of the total protein and the gammaglobulin fraction with regard to the control group. CONCLUSIONS: Higher PB19 IgG seropositivity was seen in the patients with aplastic crisis with respect to the control group, suggesting wider exposition to the virus among them with regard to the healthy population. Specific PB19 IgM was detected in 2 patients with immunodeficiency and aplastic crisis. A significant decrease of the CD4 and CD8 T-lymphocyte subsets, along with decreased CD4-CD8 quotient, were found in the aplastic crisis group, and an impairment of the immune response to the viral challenge can be inferred form this. The alterations of the serum proteins, together with the increased B lymphocytes, might suggest a polyclonal activation of these cells. The absence of other known lymphotropic viruses in most of the patients studied (50) show that the alterations found here are related to recent or past B19 infection.