Reproduction and maintenance compete for resources within a single individual. But do individuals invest in reproduction just as much as remains after the requirements of maintenance are covered, or do they sacrifice their health for the sake of still further increase the investment in current reproduction? This question has been found hard to answer because of difficulties of demonstrating that individuals naturally make a reproductive effort of such a magnitude as to inflict health damage. In this paper we present evidence for a trade-off between reproductive effort and health state in great tits, indicated by a positive correlation between total prefledging brood weight and both intensity of Haemoproteus blood parasite infection and heterophile:lymphocyte (H:L) ratio. H:L ratios, which signal stress in birds, were high both among individuals making an intense reproductive effort and among aberrantly behaving individuals, such as females incubating in empty nests and birds which abandoned their broods after blood sampling. Experimental reduction of clutch size resulted in decreased intensity of Haemoproteus parasitemia, providing further evidence that individual great tits accept immunosuppression to increase their reproductive investment.