The relationship between health and demography in the Green-rumped Parrotlet (Forpus passerinus) was examined, using hematological measurements as indicators of health. Variation in adult hematology with gender, age, time during nesting cycle, and breeding status were examined. Only gender and time during nesting cycle showed significant effects on hematology. Females had higher white-blood-cell counts and immunoglobulin-G (IgG) levels than males. Hematocrit was lower during egg laying than at other stages for females, whereas white-blood-cell counts and IgG and alpha-2-macroglobulin (α₂m) levels were higher. In males, IgG and α₂m levels were higher during egg laying. We expected that hematological indicators of health would predict reproductive success. Some relationships were identified (α₂m was negatively related to number of fledglings in males and females, female lymphocytes were negatively related to number of eggs, and basophils were negatively related to number of fledglings) at P < 0.05. However, despite substantial sample sizes (n > 25), none of those relationships were significant after Bonferroni correction, though there appears to be a threshold level for those hematological measures, below which adults may have high or low reproductive success, but above which reproductive success will be low. We suggest that the effect of micro-rather than macroparasites may affect health, resulting in weak effects on reproductive success. Because of the transient nature of microparasitic infection, health and reproductive success may be more significantly related over an organism’s lifetime than in any given year.