Although the influence of the levels and ratios of the primary stimulus on the amplitude of distortion product otoacoustic emissions (DPOAEs) has been studied intensely, the influence of the presence of spontaneous otoacoustic emissions (SOAEs) has been investigated less thoroughly. The present investigation analysed whether the unilateral presence of 58 SOAEs in 43 normal-hearing adults was related to larger DPOAEs in the ear with SOAEs compared to the contralateral ear having no SOAEs. The study was designed such that the only factor that could influence the amplitude of DPOAEs was the presence of SOAEs. Input/output (IIO) functions were collected in response to primary tones that were presented in 5-dB steps from 70 to 40 dB SPL at the frequency of the unilaterally recorded SOAE of each subject. The primary outcome was the demonstration of statistically significant (P < 0.05) larger DPOAEs in ears exhibiting SOAEs than in ears without measurable SOAEs, except at the highest stimulus level of 70 dB SPL. These results suggest that SOAEs play an additive role in the measurement of DPOAEs. The enhancing effect of the unilateral presence of SOAEs on DPOAEs was statistically significant for 65 dB SPL and lower levels of primary tones. The authors speculate that passive cochlear properties begin to participate in the generation of DPOAEs at primary-stimulus levels greater than 65 dB SPL. (C) 1999 Elsevier Science B.V. All rights reserved.