To examine whether use of insulin glargine, compared with another long-acting insulin, is associated with risk of breast, prostate, colorectal cancer, or all cancers combined. Computerized health records from Kaiser Permanente Northern and Southern California regions starting in 2001 and ending in 2009 were used to conduct a population-based cohort study among patients with diabetes aged ?18 years. With use of Cox regression modeling, cancer risk in users of insulin glargine (n = 27,418) was compared with cancer risk in users of NPH (n = 100,757). The cohort had a median follow-up of 3.3 years during which there was a median of 1.2 years of glargine use and 1.4 years of NPH use. Among users of NPH at baseline, there was no clear increase in risk of breast, prostate, colorectal, or all cancers combined associated with switching to glargine. Among those initiating insulin, ever use or ?2 years of glargine was not associated with increased risk of prostate or colorectal cancer or all cancers combined. Among initiators, the hazard ratio (HR) for breast cancer associated with ever use of glargine was 1.3 (95% CI 1.0-1.8); the HR for breast cancer associated with use of glargine for ?2 years was 1.6 or 1.7 depending on whether glargine users had also used NPH. Results of this study should be viewed cautiously, given the relatively short duration of glargine use to date and the large number of potential associations examined.