Clozapine and caffeine are metabolised mainly by the cytochrome P4501A2 (CYP1A2) enzyme. Studies suggest that caffeine in coffee inhibits clozapine metabolism and increases serum clozapine concentrations. Our objective was to study whether coffee in the amounts usually consumed has an effect on steady-state serum clozapine concentrations. A randomised placebo-controlled cross-over design with two phases was used. Twelve hospitalised clozapine-using patients volunteered in the study where, after one-week run-in period, either caffeine-containing or decaffeinated instant coffee was available ad libitum for seven days. Serum concentrations of clozapine, N-desmethylclozapine, clozapine-N-oxide, caffeine, paraxanthine and C-reactive protein were measured after run-in period and on days 4 and 8 of the following study phases. Two patients were excluded from the statistical analysis because of non-compliance based on serum caffeine and paraxanthine determinations. In six fully compliant patients caffeine-containing coffee increased the mean serum trough concentration of clozapine by 26% (non-significant (NS), 95% CI -3% to +54%, P=0.07), N-desmethylclozapine by 6% (95% CI 1% to 12%, P=0.03), and clozapine-N-oxide by 7% (NS, 95% CI -6% to +20%, P=0.22). The ratio of N-desmethylclozapine/clozapine decreased by 13% (NS, 95% CI -1% to +27%, P=0.06) and that of clozapine-N-oxide/clozapine by 7% (NS, 95% CI -5% to +17%, P=0.19). In the analysis of combined data (including day 4 data of the four patients compliant up to that point) serum trough concentration of clozapine was 20% (95% CI 3% to 37% to P=0.03) higher, and that of N-desmethylclozapine 7% (95% CI 2% to 13%, P=0.02) higher during the caffeine phase than during the decaffeinated phase. We conclude that the effect of instant coffee drinking on serum clozapine concentrations is of minor clinical relevance in most of the patients, but some individuals may be more sensitive to this interaction due e.g. to genetic factors. The increase in serum clozapine concentration was most likely due to the inhibition of the CYP1A2 enzyme by caffeine.