This study investigates human environmental adaptation in northern Finland at the cemetery of Iin Hamina. The cemetery was in use in the 15th and 16th centuries AD during the cool climate anomaly called the Little Ice Age. It is possible that these extreme climatic conditions may have impacted human survival in this agriculturally marginal region. In previous studies, carbon and nitrogen isotopes in the bone collagen of humans and fauna have shown that the main protein source for people at Iin Hamina was fish. In this study, I observe annual changes in diet derived from isotopic studies of teeth. Apart from one individual whose protein source shifted radically during tooth formation, the results show the inhabitants of Iin Hamina experienced only minor changes in diet over periods of several years. Moreover, none of the dietary profiles provided evidence of dietary or physiological stress through raised nitrogen isotope ratios. The results indicate that the individuals in this study appear to have been well-adapted to their environment.