Healthy feline leukemia virus (FeLV)-infected cats from leukemia cluster environments were followed for up to 23 months for development of disease and evidence of alteration in the hemogram. The incidence of disease development in FeLV-postive cats was more than fivefold higher than the incidence for FeLV-negative cats. Ten cases of leukemia developed in 69 infected cats, whereas one case of leukemia occurred in 59 uninfected cats. The incidence for development of diseases other than leukemia was 30.4 percent for FeLV-infected cats as opposed to 6.8 per cent for uninfected cats. This could be a result of the immunosuppressive effects of FeLV. Felv-infected cats had no evidence of subclinical anemia. Mean packed cell volumes and total leukocyte counts were about the same for infected and uninfected animals. The only variation seen in healthy FeLV-infected cats was a decreased mean lymphocyte count. The difference between mean lymphocyte count for FeLV-infected and uninfected animals was significant at the 0.999 level. These findings suggest that the incubation period for feline leukemia may be very prolonged under natural conditions and that an increased susceptibility to unrelated infectious diseases exists during this period. This increased susceptibility was apparently not associated with anemia or depressed total leukocyte counts.