Fluorescence cytophotometric DNA measurements on 15 carcinoid tumours generally yielded histograms with a prominent G1-phase peak and only low frequencies of S- and G2-phase nuclei. In 13 tumours the modal DNA value was near diploid (1.75-2.25). Three of these 15 tumours showed a much greater number of S- and G2-phase values than the others and in two of them there were also G1-phase peaks below the near-diploid range. Among 22 samples of small cell carcinoma of the lung (SCCL), the modal DNA value was hypodiploid in two, near diploid in seven and hyperdiploid in 13. The proportion of S-phase cells was higher in the SCCL samples than in the carcinoid tumours. It is suggested that the nuclear DNA distribution profile may facilitate morphological discrimination of a classical pulmonary carcinoid from small cell carcinoma. In a borderline case, the finding of a hyperdiploid DNA stem line and high proliferative activity will favour the diagnosis of SCCL.