Use of permanent hair dye has been suggested as a risk factor for several types of cancer, although epidemiologic data have not generally supported this hypothesis. Retrospective studies have reported a possible association between hair dyes and hematopoietic cancers. Purpose : Our purpose was to investigate if permanent hair dye was associated with risks of incident lymphoma, leukemia, and multiple myeloma in the Nurses' Health Study, a prospective cohort study of 99 067 women aged 30–55 years in 1976. Methods : Questionnaires regarding medical history and other health-related variables were sent to Nurses' Health Study participants every 2 years from 1976 to 1990. The follow-up for mortality in this cohort exceeds 98%. We identified 244 newly diagnosed cases of hematopoietic cancers, confirmed by pathology reports. Permanent hair dye use was ascertained over four cycles of questionnaires from 1976–1982; status of hair dye use established in 1982 was then used for the remainder of the follow-up time (through 1990). Age-specific incidence rates were calculated and used to compute relative risks (RRs) with 95% confidence intervals (CIs). Results : We found no evidence of a positive association between ever use of permanent hair dye and all hematopoietic cancers (age-adjusted RR = 0.9; 95% CI = 0.7–1.2) or specific types (Hodgkin's lymphoma [RR = 0.9; 95% CI = 0.4–2.1], non-Hodgkin's lymphoma [RR – 1.1; 95% CI = 0.8–1.6], multiple myeloma [RR = 0.4; 95% CI = 0.2–0.9], chronic lymphocytic leukemia [RR = 0.6; 95% CI = 0.3–1.5], and other leukemias [RR = 0.8; 95% CI = 0.3–1.9). Further examination of age at first use, duration, frequency, and time since first use and risk of all hematopoietic cancers or non-Hodgkins; lymphoma (the largest diagnostic group), indicated no material associations. Conclusion : In this prospective cohort study, permanent hair dye use is not adversely related to risks of hematopoietic cancers. (J. Natl Cancer Inst 86: 1466–1470, 1994)