Background: Treatment of drug addicts is one of the main strategies of drug control in Iran. Client satisfaction strongly influences the success of any treatment program. This study aimed to explore the difference between customer expectations and perceptions in drug addiction treatment centers of Kerman, Iran, using SERVQUAL model. Methods: Using a cross-sectional design 260 clients referring to drug addiction treatment centers of Kerman, Iran, were enrolled in 2012. From among 84 clinics, 20 centers were selected randomly. Based on the number of clients registered in each center, a random sample proportional to the size was selected and 290 subjects were invited for interviews. A well validated 22-item questionnaire, which measured the 5 dimensions of service quality (reliability, assurance, tangibility, empathy, and responsiveness), was completed by participants. Each item measured 2 aspects of service quality; expectations and perceptions. Findings: Mean ± SD (Standard deviation) age of the subjects was 37.7 ± 9.4. Most of them were male (87.7%). Less than half of them had an educational level lower than diploma. The total score of clients` expectations was higher than their perceptions (P < 0.001). Considering the 5 dimensions of the SERVQUAL model, only 1 dimension (i.e., assurance) showed no difference between perceptions and expectations of the participants (P = 0.134). Conclusion: There was a gap between the clients’ expectations and what they actually perceived in the clinics. Thus, more attention should be devoted to the clients` views regarding service quality in addiction treatment clinics.