There are few published reports on cancer incidence in the Puerto Rican-born populations of the northeastern United States. In Connecticut, in the Puerto Rican-born population, which was of low socioeconomic status (i.e., 42% below the poverty level in the 1980 Census), the standardized incidence ratio (SIR) for all invasive cancers diagnosed in 1980-1986 was significantly reduced for female patients (SIR = 0.77) but not for male patients (SIR = 1.16), on the basis of expected numbers derived from incidence rates for the entire state of Connecticut. For female patients, only the SIR for cancers of the stomach, esophagus, and cervix were elevated significantly, whereas those for colorectal, lung, breast, and ovarian cancer were significantly reduced. For male patients, SIR were elevated significantly for cancer of the oral cavity, esophagus, and stomach and for leukemia, whereas none of the sites (including lung) had significantly reduced SIR. When incidence rates for Puerto Rico were used, the SIR for all sites combined was 1.99 (95% confidence interval = 1.78-2.22) for male patients but only 1.39 (95% confidence interval = 1.24-1.56) for female patients. These findings suggest sex differences in acculturation and lifestyle changes relevant to cancer risks in immigrants from Puerto Rico residing in Connecticut. Comparisons were made with cancer incidence and mortality data from other Puerto Rican immigrant populations.