Objective: Evidence for a clinically significant effect of acclimatization to hearing aids is mixed. The aim of this study was to test for auditory acclimatization effects in new unilateral and bilateral adult hearing aid users. Hypotheses were i) there would be improvements in aided speech recognition in new hearing aid users, compared with unaided listening and with a control group of experienced hearing aid users, and ii) improvements would correlate with severity of hearing loss, hearing aid use, and cognitive capacity. Design: Speech recognition in noise was measured for a 65 and a 75 dB SPL target with the Four Alternative Auditory Feature test. Speech recognition in noise was measured within 1 week of fitting and retested at 12 weeks postfitting in new hearing aid users (16 unilateral and 16 bilateral fit). A control group of experienced hearing aid users (n = 17) was tested over a similar time scale. Cognitive capacity (reaction time and working memory) was measured, and self-reported change in performance was assessed using the Speech, Spatial and Qualities of Hearing Scale. Hearing aid use was assessed via data logging at the completion of the study. Results: Mean improvements in speech recognition of up to 4% were observed across conditions and across groups consistent with a general practice effect. On average there was no evidence of auditory acclimatization in the new hearing aid user groups in terms of improvement in aided listening conditions above that observed in unaided recognition or in the control group. There was no correlation between change in aided speech recognition and severity of hearing loss, hearing aid use, or cognitive capacity. New users reported significant improvement over time in aided performance on a self-report questionnaire compared with the control group. Conclusions: On average, there was no improvement over time in new usersâ€™ aided speech recognition relative to unaided recognition or to the control group. This does not support a robust acclimatization effect with nonlinear hearing aids. Testâ€“retest variability may obscure small average acclimatization effects; variability was not accounted for by individual differences in severity of hearing loss, hearing aid use, or cognitive capacity. New usersâ€™ subjective report of increased benefit over time may be reflective of other aspects of adjustment to hearing aid use not examined in this study.