The relationship of weight at birth to the occurrence of childhood cancer was studied with emphasis on the influence of age at diagnosis. Birth certificates for 681 children with cancer born in Washington State were linked with cancer registry data. Among children diagnosed with cancer during the first several years of life, there was an increased proportion with a high birth weight (greater than 4,000 g). The relationship was strongest for children under 2 years of age; about twice as many of them had high birth weights. However, the relationship was not present at all in those whose cancer was diagnosed at age 4 or older. This excess risk in young children associated with high birth weight was distributed among several types of cancer, including the two most common ones (leukemia and neuroblastoma).