Compliance of 21 private medical laboratories in Osun State with ethical and other professional was assessed by the authors' pre and post inspection by the Medical Laboratory Science Council of Nigeria (MLSCN). Laboratory environment, personnel, equipment and adherence to Standard Operating Procedures (SOPs) were used as indicators and the results obtained were compared. Thirteen (62 %) was found to have conformed to environmental specifications before the MLSCN's inspection. This rose to 84% and 89% a month and 6 months post MLSCN inspection respectively. Ten (48%) had the right personnel before the Council's inspection followed by 74% which dropped to 72% six months later. Fifteen (67%) laboratories had prerequisite equipment in use before the Council's inspection and this rose to 84% and 89% a month and 6 months later, respectively. Adherence to SOPs was observed to have risen from 76% baseline to 95% a month after; the value dropped to 89% 6 months after the Council's inspection. Follow-up assessment by volunteer members of the Association of Medical Laboratory Scientists of Nigeria noted a sustained improvement of professional standards in private medical laboratories till 6 months after MLSCN inspection. It is recommended that competent volunteers be engaged by the MLSCN and other stakeholders within and outside the country to continually monitor the sustenance of acceptable standards of practice by medical laboratories.Keywords: Ethics, private medical laboratory, service