He observed that the rate of skin cancer in Australia among Caucasians was strongly correlated with latitude, and hence with amount of sunlight; the northern states had higher rates than the southern ones. This was well before the hole in the ozone layer!
This observation was only possible by careful collection of data, and comparison of skin cancer rates.
Lancaster also made an important discovery about German measles and birth defects .
Lancaster, H. O. (1956) Some geographical aspects of the mortality from melanoma in Europeans. Medical Journal of Australia , 1 : 1082-1087.