Motivational interviewing is a client centred behavioural therapy for addictive behaviours. It is an intervention designed to help all addicts, not just those ready to change. It is therefore suitable for use as an opportunistic intervention for clients whose main reason for contact may not be their addiction. A pilot randomized controlled trial of home-based motivational interviewing by a specially trained midwife to help pregnant smokers reduce their habit was performed in Glasgow from February 1997 to January 1998. Did motivational interviewing take place? All 171 counselling interviews from 48 intervention clients were audio-taped. Forty-nine interviews from 13 randomly selected clients were transcribed for content analysis. A rating scale established for feedback to trainee psychologists was used by three experienced analysts. Thirty-two interviews were scored independently to validate the rating scale in this setting. More than 75% of interviews showed satisfactory motivational interviewing. Therapist utterances were motivational, and client responses included many self-motivational statements. Few episodes of client resistance were recorded. Rating took 160 mins per half hour interview. This instrument provided a valid measure of intervention quality for a randomized controlled trial. It would not be practical to document process outside a research environment.