A phase I trial was performed to investigate the tolerability and efficacy of the novel nucleoside analogue gemcitabine in combination with cisplatin in the treatment of advanced non-small cell lung cancer. Both cisplatin and gemcitabine were administered as 30 min infusions weekly x 3 with a week of rest. There was one dose escalation of cisplatin from 25 mg/m2 (dose level 1) to 30 mg/m2 (in subsequent dose levels 2-5), such that the mean dose intensity for the weekly x 3 q 4 week cycle was 22.5 mg/m2/week which is close to that achieved with 100 mg/m2 bolus monthly. Gemcitabine was initiated at 1000 mg/m2 (dose levels 1 and 2) then escalated by 250 mg/m2/week to 1750 mg/m2 (dose level 5). Of 32 chemotherapy-naive patients entered (22 males, 10 females; median age 61 years, range 29-74 years), 11 had localized tumours (2 stage IIIa, 9 IIIb) and 21 had stage IV tumours with haematogenous metastases and a poor prognosis. Twenty-one patients had adenocarcinoma, 4 squamous cell carcinoma, 6 large cell undifferentiated tumors, and one had mixed squamous and adenocarcinoma. Dose-limiting toxicity was not seen in more than one patient in cycle 1 at any dose level. Grade 4 granulocytopenia and thrombocytopenia occurred more frequently with repeated dosing, necessitating dose reductions except at the lowest dose level (cisplatin 25 mg/m2, gemcitabine 1000 mg/m2). Non-haematological toxicity was mild and rapidly reversible. Cisplatin administration led to a higher frequency of nausea and vomiting than that seen with gemcitabine alone, but this was easily controlled with antiemetics. In the 28 patients evaluable, to date responses have been seen at most dose levels, with an overall response rate 35.7%. This phase I trial is ongoing and further dose escalation is intended to determine the MTD of gemcitabine.