BACKGROUND: Poor quality of care, including overprovision (unnecessary care) is a global health concern. Greater provider effort has been shown to increase the likelihood of correct treatment, but its relationship with overprovision is less clear. Providers who make more effort may give more treatment overall, both correct and unnecessary, or may have lower rates of overprovision; we test which is true in the Tanzanian private health sector. METHODS: Standardised patients visited 227 private-for-profit and faith-based facilities in Tanzania, presenting with symptoms of asthma and TB. They recorded history questions asked and physical examinations carried out by the provider, as well as laboratory tests ordered, treatments prescribed, and fees paid. A measure of provider effort was constructed on the basis of a checklist of recommended history taking questions and physical exams. RESULTS: 15% of SPs received the correct care for their condition and 74% received unnecessary care. Increased provider effort was associated with increased likelihood of correct care, and decreased likelihood of giving unnecessary care. Providers who made more effort charged higher fees, through the mechanism of higher consultation fees, rather than increased fees for lab tests and drugs. CONCLUSION: Providers who made more effort were more likely to treat patients correctly. A novel finding of this study is that they were also less likely to provide unnecessary care, suggesting it is not simply a case of some providers doing "more of everything", but that those who do more in the consultation give more targeted care.