This cohort study examined cancer incidence in 13,650 Connecticut cosmetologists who had held licenses for 5 years or more and had begun hairdressing school prior to January 1, 1966. Cancer incidence rates for the general Connecticut population, 1935-78, were compared with those for 11,845 female and 1,805 male cosmetologists. The females had a standardized cancer incidence ratio (SIR) of 112 (P less than .01). A significant excess of lung cancer (SIR = 141) and excesses of brain (SIR = 168) and ovarian cancer (SIR = 134) of borderline significance were observed. No significant cancer risk was evident for female cosmetologists licensed since 1935, even for those with 35 years or more of follow-up, although the SIRs for brain cancer, lymphoma, and leukemia were elevated. Female cosmetologists who entered the profession between 1925 and 1934, however, experienced a significant overall cancer incidence (SIR = 129) and significant excesses of respiratory, breast, corpus uterine, and ovarian cancers. Those with 35 years or more from time of first license appeared to be at the highest risk. Among males the overall cancer incidence rate was close to that expected (SIR = 105). Smoking habits and reproductive factors that could not be taken into account may explain some of the excesses among females. Although no specific occupational agent could be identified, the excess numbers of leukemias in females and brain cancers among males and females merit continued surveillance.